diff --git a/.vscode/settings.json b/.vscode/settings.json
index c37723d..d3e22f1 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -1,8 +1,13 @@
 {
   "cSpell.words": [
+    "andalso",
     "backoff",
     "casted",
+    "Desugars",
     "mappish",
-    "Planable"
+    "noreply",
+    "Planable",
+    "splode",
+    "Splode"
   ]
 }
diff --git a/lib/reactor.ex b/lib/reactor.ex
index dd054c2..a058df8 100644
--- a/lib/reactor.ex
+++ b/lib/reactor.ex
@@ -143,7 +143,8 @@ defmodule Reactor do
     if Spark.Dsl.is?(reactor, Reactor) do
       run(reactor.reactor(), inputs, context, options)
     else
-      {:error, "Module `#{inspect(reactor)}` is not a Reactor module"}
+      {:error,
+       ArgumentError.exception(message: "Module `#{inspect(reactor)}` is not a Reactor module")}
     end
   end
 
diff --git a/lib/reactor/builder/compose.ex b/lib/reactor/builder/compose.ex
index b17cf62..ca2c3fb 100644
--- a/lib/reactor/builder/compose.ex
+++ b/lib/reactor/builder/compose.ex
@@ -11,7 +11,7 @@ defmodule Reactor.Builder.Compose do
   import Reactor, only: :macros
   import Reactor.Argument, only: :macros
   import Reactor.Utils
-  alias Reactor.{Argument, Builder, Error.ComposeError, Step}
+  alias Reactor.{Argument, Builder, Error.Internal.ComposeError, Step}
 
   @doc """
   Compose another Reactor inside this one.
diff --git a/lib/reactor/error.ex b/lib/reactor/error.ex
new file mode 100644
index 0000000..d52cf8d
--- /dev/null
+++ b/lib/reactor/error.ex
@@ -0,0 +1,24 @@
+defmodule Reactor.Error do
+  @moduledoc """
+  Uses `splode` to manage various classes of error.
+  """
+
+  use Splode,
+    error_classes: [
+      reactor: Reactor.Error.Internal,
+      invalid: Reactor.Error.Invalid,
+      unknown: Reactor.Error.Unknown,
+      validation: Reactor.Error.Validation
+    ],
+    unknown_error: Reactor.Error.Unknown.UnknownError
+
+  @doc "Convenience wrapper around `use Splode.Error`"
+  @spec __using__(keyword) :: Macro.output()
+  defmacro __using__(opts) do
+    quote do
+      use Splode.Error, unquote(opts)
+      import Reactor.Error.Utils
+      import Reactor.Utils
+    end
+  end
+end
diff --git a/lib/reactor/error/internal.ex b/lib/reactor/error/internal.ex
new file mode 100644
index 0000000..0acfbcb
--- /dev/null
+++ b/lib/reactor/error/internal.ex
@@ -0,0 +1,13 @@
+defmodule Reactor.Error.Internal do
+  @moduledoc """
+  The [Splode error class](e:splode:get-started-with-splode.html#error-classes)
+  for Reactor-caused errors.
+  """
+  use Reactor.Error, fields: [:errors], class: :reactor
+
+  @doc false
+  @impl true
+  def splode_message(%{errors: errors}) do
+    Splode.ErrorClass.error_messages(errors)
+  end
+end
diff --git a/lib/reactor/errors/compose_error.ex b/lib/reactor/error/internal/compose_error.ex
similarity index 68%
rename from lib/reactor/errors/compose_error.ex
rename to lib/reactor/error/internal/compose_error.ex
index 5c40046..130a161 100644
--- a/lib/reactor/errors/compose_error.ex
+++ b/lib/reactor/error/internal/compose_error.ex
@@ -1,20 +1,18 @@
-defmodule Reactor.Error.ComposeError do
+defmodule Reactor.Error.Internal.ComposeError do
   @moduledoc """
-  An error used when attempting to compose to Reactors together.
+  This error is returned when two Reactors cannot be composed together.
   """
-  defexception [:outer_reactor, :inner_reactor, :message, :arguments]
-  import Reactor.Utils
+
+  use Reactor.Error,
+    fields: [:arguments, :inner_reactor, :message, :outer_reactor],
+    class: :reactor
 
   @doc false
   @impl true
-  def exception(attrs), do: struct(__MODULE__, attrs)
-
-  @doc false
-  @impl true
-  def message(error) do
+  def splode_message(error) do
     [
       """
-      # Unable to compose Reactors
+      # Reactor Compose Error
 
       #{error.message}
       """
diff --git a/lib/reactor/error/internal/missing_return_result_error.ex b/lib/reactor/error/internal/missing_return_result_error.ex
new file mode 100644
index 0000000..85f5f18
--- /dev/null
+++ b/lib/reactor/error/internal/missing_return_result_error.ex
@@ -0,0 +1,33 @@
+defmodule Reactor.Error.Internal.MissingReturnResultError do
+  @moduledoc """
+  This error is returned when the Reactor's return name doesn't match any of the
+  known step results.
+  """
+
+  use Reactor.Error,
+    fields: [:reactor],
+    class: :reactor
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    intermediate_keys =
+      error.reactor.intermediate_values
+      |> Map.keys()
+
+    known_results =
+      intermediate_keys
+      |> Enum.map_join("\n", &"  * `#{inspect(&1)}`")
+
+    """
+    # Missing Return Result Error
+
+    The Reactor was asked to return a result named `#{inspect(error.reactor.return)}`, however an intermediate result with that name is missing.
+    #{did_you_mean?(error.reactor.return, intermediate_keys)}
+
+    ## Intermediate results:
+
+    #{known_results}
+    """
+  end
+end
diff --git a/lib/reactor/errors/plan_error.ex b/lib/reactor/error/internal/plan_error.ex
similarity index 66%
rename from lib/reactor/errors/plan_error.ex
rename to lib/reactor/error/internal/plan_error.ex
index 930457a..d3e171c 100644
--- a/lib/reactor/errors/plan_error.ex
+++ b/lib/reactor/error/internal/plan_error.ex
@@ -1,20 +1,20 @@
-defmodule Reactor.Error.PlanError do
+defmodule Reactor.Error.Internal.PlanError do
   @moduledoc """
-  An error thrown during the planning of a Reactor.
+  This error is returned when the step graph cannot be built.
   """
-  defexception [:reactor, :graph, :step, :message]
-  import Reactor.Utils
+
+  use Reactor.Error,
+    fields: [:graph, :message, :reactor, :step],
+    class: :reactor
 
   @doc false
   @impl true
-  def exception(attrs), do: struct(__MODULE__, attrs)
-
-  @doc false
-  @impl true
-  def message(error) do
+  def splode_message(error) do
     [
       """
-      # Unable to plan Reactor
+      # Reactor Plan Error
+
+      An error occurred while building or updating the Reactor execution graph.
 
       #{error.message}
       """
diff --git a/lib/reactor/error/invalid.ex b/lib/reactor/error/invalid.ex
new file mode 100644
index 0000000..dcf9768
--- /dev/null
+++ b/lib/reactor/error/invalid.ex
@@ -0,0 +1,14 @@
+defmodule Reactor.Error.Invalid do
+  @moduledoc """
+  The [Splode error class](e:splode:get-started-with-splode.html#error-classes)
+  for user-caused errors.
+  """
+
+  use Reactor.Error, fields: [:errors], class: :unknown
+
+  @doc false
+  @impl true
+  def splode_message(%{errors: errors}) do
+    Splode.ErrorClass.error_messages(errors)
+  end
+end
diff --git a/lib/reactor/error/invalid/argument_subpath_error.ex b/lib/reactor/error/invalid/argument_subpath_error.ex
new file mode 100644
index 0000000..3571e1a
--- /dev/null
+++ b/lib/reactor/error/invalid/argument_subpath_error.ex
@@ -0,0 +1,38 @@
+defmodule Reactor.Error.Invalid.ArgumentSubpathError do
+  @moduledoc """
+  This error is returned when an argument cannot have a subpath applied to it.
+  """
+  use Reactor.Error,
+    fields: [:argument, :culprit, :culprit_key, :culprit_path, :message, :step, :value],
+    class: :invalid
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    """
+    # Argument Subpath Error
+
+    The step `#{inspect(error.step.name)}` is expecting the value for argument `#{inspect(error.argument.name)}` to be able to be subpathed via `#{inspect(error.argument.subpath)}` however #{error.reason}.
+
+    ## `step`:
+
+    #{inspect(error.step)}
+
+    ## `argument`:
+
+    #{inspect(error.argument)}
+
+    ## `value`:
+
+    #{inspect(error.value)}
+
+    ## `culprit`:
+
+    #{inspect(error.culprit)}
+
+    ## `culprit_path`:
+
+    #{inspect(error.culprit_path)}
+    """
+  end
+end
diff --git a/lib/reactor/error/invalid/compensate_step_error.ex b/lib/reactor/error/invalid/compensate_step_error.ex
new file mode 100644
index 0000000..f6742d8
--- /dev/null
+++ b/lib/reactor/error/invalid/compensate_step_error.ex
@@ -0,0 +1,28 @@
+defmodule Reactor.Error.Invalid.CompensateStepError do
+  @moduledoc """
+  This error is returned when an error occurs during step compensation.
+
+  Its `error` key will contain the error that was raised or returned by the
+  `c:Step.compensate/4` callback.
+  """
+
+  use Reactor.Error, fields: [:error, :reactor, :step], class: :invalid
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    """
+    # Compensate Step Error
+
+    An error occurred while attempting to compensate the `#{inspect(error.step.name)}` step.
+
+    ## `step`:
+
+    #{inspect(error.step)}
+
+    ## `error`:
+
+    #{describe_error(error.error)}
+    """
+  end
+end
diff --git a/lib/reactor/error/invalid/missing_argument_error.ex b/lib/reactor/error/invalid/missing_argument_error.ex
new file mode 100644
index 0000000..7896492
--- /dev/null
+++ b/lib/reactor/error/invalid/missing_argument_error.ex
@@ -0,0 +1,32 @@
+defmodule Reactor.Error.Invalid.MissingArgumentError do
+  @moduledoc """
+  This error is returned when an expected argument is not passed to a step.
+  """
+
+  use Reactor.Error, fields: [:argument, :arguments, :step], class: :invalid
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    """
+    # Missing Argument Error
+
+    The step `#{inspect(error.step.name)}` run function is expecting to be passed the `#{inspect(error.argument)}` argument, but it is not present.
+    #{did_you_mean?(error.argument, Map.keys(error.arguments))}
+
+    ## `step`:
+
+    #{inspect(error.step)}
+
+    ## `argument`:
+
+    #{inspect(error.argument)}
+
+    ## Arguments passed:
+
+    ```
+    #{inspect(error.arguments)}
+    ```
+    """
+  end
+end
diff --git a/lib/reactor/error/invalid/missing_input_error.ex b/lib/reactor/error/invalid/missing_input_error.ex
new file mode 100644
index 0000000..fb1bebf
--- /dev/null
+++ b/lib/reactor/error/invalid/missing_input_error.ex
@@ -0,0 +1,34 @@
+defmodule Reactor.Error.Invalid.MissingInputError do
+  @moduledoc """
+  Error raised when a required Reactor input is missing.
+  """
+
+  use Reactor.Error, fields: [:argument, :reactor, :step], class: :invalid
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    inputs =
+      error.reactor.inputs
+      |> Enum.map_join("\n", &"  * `#{inspect(&1)}`")
+
+    """
+    # Missing Input Error
+
+    The step `#{inspect(error.step.name)}` is expecting the Reactor to have an input named `#{inspect(error.argument.source.name)}` however it is not present.
+    #{did_you_mean?(error.argument.source.name, error.reactor.inputs)}
+
+    ## `step`:
+
+    #{inspect(error.step)}
+
+    ## `argument`:
+
+    #{inspect(error.argument)}
+
+    ## Available inputs:
+
+    #{inputs}
+    """
+  end
+end
diff --git a/lib/reactor/error/invalid/missing_result_error.ex b/lib/reactor/error/invalid/missing_result_error.ex
new file mode 100644
index 0000000..c59d712
--- /dev/null
+++ b/lib/reactor/error/invalid/missing_result_error.ex
@@ -0,0 +1,34 @@
+defmodule Reactor.Error.Invalid.MissingResultError do
+  @moduledoc """
+  This error is returned when a step attempts to consume an intermediate result
+  which is not present in the Reactor state.
+  """
+  use Reactor.Error, fields: [:argument, :reactor, :step], class: :invalid
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    inputs =
+      error.reactor.inputs
+      |> Enum.map_join("\n", &"  * `#{inspect(&1)}`")
+
+    """
+    # Missing Result Error
+
+    The step `#{inspect(error.step.name)}` is expecting the Reactor to have an existing result named `#{inspect(error.argument.source.name)}` however it is not present.
+    #{did_you_mean?(error.argument.source.name, Map.keys(error.reactor.intermediate_results))}
+
+    ## `step`:
+
+    #{inspect(error.step)}
+
+    ## `argument`:
+
+    #{inspect(error.argument)}
+
+    ## Available inputs:
+
+    #{inputs}
+    """
+  end
+end
diff --git a/lib/reactor/error/invalid/retries_exceeded_error.ex b/lib/reactor/error/invalid/retries_exceeded_error.ex
new file mode 100644
index 0000000..2c7d26a
--- /dev/null
+++ b/lib/reactor/error/invalid/retries_exceeded_error.ex
@@ -0,0 +1,25 @@
+defmodule Reactor.Error.Invalid.RetriesExceededError do
+  @moduledoc """
+  This error is returned when a step attempts to retry more times that is
+  allowed.
+  """
+  use Reactor.Error, fields: [:retry_count, :step], class: :invalid
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    """
+    # Retries Exceeded Error
+
+    Maximum number of retries exceeded executing step `#{inspect(error.step.name)}`.
+
+    ## `retry_count`:
+
+    #{inspect(error.retry_count)}
+
+    ## `step`:
+
+    #{inspect(error.step)}
+    """
+  end
+end
diff --git a/lib/reactor/error/invalid/run_step_error.ex b/lib/reactor/error/invalid/run_step_error.ex
new file mode 100644
index 0000000..3951a81
--- /dev/null
+++ b/lib/reactor/error/invalid/run_step_error.ex
@@ -0,0 +1,27 @@
+defmodule Reactor.Error.Invalid.RunStepError do
+  @moduledoc """
+  This error is returned when an error occurs during step execution.
+
+  Its `error` key will contain the error that was raised or returned by the
+  `c:Step.run/3` callback.
+  """
+  use Reactor.Error, fields: [:error, :step], class: :invalid
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    """
+    # Run Step Error
+
+    An error occurred while attempting to run the `#{inspect(error.step.name)}` step.
+
+    ## `step`:
+
+    #{inspect(error.step)}
+
+    ## `error`:
+
+    #{describe_error(error.error)}
+    """
+  end
+end
diff --git a/lib/reactor/error/invalid/transform_error.ex b/lib/reactor/error/invalid/transform_error.ex
new file mode 100644
index 0000000..f619866
--- /dev/null
+++ b/lib/reactor/error/invalid/transform_error.ex
@@ -0,0 +1,41 @@
+defmodule Reactor.Error.Invalid.TransformError do
+  @moduledoc """
+  An error which occurs when building and running transforms.
+  """
+  use Reactor.Error, fields: [:input, :output, :error], class: :invalid
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    message = """
+    # Transform Error
+
+    An error occurred while trying to transform a value.
+
+    ## `input`:
+
+    `#{inspect(error.input)}`
+    """
+
+    message =
+      if error.output do
+        """
+        #{message}
+
+        ## `output`:
+
+        `#{inspect(error.output)}`
+        """
+      else
+        message
+      end
+
+    """
+    #{message}
+
+    ## `error`:
+
+    #{describe_error(error.error)}
+    """
+  end
+end
diff --git a/lib/reactor/error/invalid/undo_retries_exceeded.ex b/lib/reactor/error/invalid/undo_retries_exceeded.ex
new file mode 100644
index 0000000..a87b3d2
--- /dev/null
+++ b/lib/reactor/error/invalid/undo_retries_exceeded.ex
@@ -0,0 +1,25 @@
+defmodule Reactor.Error.Invalid.UndoRetriesExceededError do
+  @moduledoc """
+  An error used when a step runs out of retry events and no other error is
+  thrown.
+  """
+  use Reactor.Error, fields: [:step, :retry_count], class: :invalid
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    """
+    # Undo Retries Exceeded Error
+
+    Maximum number of retries exceeded while attempting to undo step `#{inspect(error.step.name)}`.
+
+    ## `retry_count`:
+
+    #{inspect(error.retry_count)}
+
+    ## `step`:
+
+    #{inspect(error.step)}
+    """
+  end
+end
diff --git a/lib/reactor/error/invalid/undo_step_error.ex b/lib/reactor/error/invalid/undo_step_error.ex
new file mode 100644
index 0000000..c4728c4
--- /dev/null
+++ b/lib/reactor/error/invalid/undo_step_error.ex
@@ -0,0 +1,27 @@
+defmodule Reactor.Error.Invalid.UndoStepError do
+  @moduledoc """
+  This error is returned when an error occurs when attempting to undo step execution.
+
+  Its `error` key will contain the error that was raised or returned by the
+  `c:Step.undo/4` callback.
+  """
+  use Reactor.Error, fields: [:step, :error], class: :invalid
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    """
+    # Undo Step Error
+
+    An error occurred while attempting to undo the step `#{inspect(error.step.name)}`.
+
+    ## `step`:
+
+    #{inspect(error.step)}
+
+    ## `error`:
+
+    #{describe_error(error.error)}
+    """
+  end
+end
diff --git a/lib/reactor/error/unknown.ex b/lib/reactor/error/unknown.ex
new file mode 100644
index 0000000..21366fe
--- /dev/null
+++ b/lib/reactor/error/unknown.ex
@@ -0,0 +1,14 @@
+defmodule Reactor.Error.Unknown do
+  @moduledoc """
+  The [Splode error class](e:splode:get-started-with-splode.html#error-classes)
+  for unknown errors.
+  """
+
+  use Reactor.Error, fields: [:errors], class: :unknown
+
+  @doc false
+  @impl true
+  def splode_message(%{errors: errors}) do
+    Splode.ErrorClass.error_messages(errors)
+  end
+end
diff --git a/lib/reactor/error/unknown/unknown.ex b/lib/reactor/error/unknown/unknown.ex
new file mode 100644
index 0000000..8851b50
--- /dev/null
+++ b/lib/reactor/error/unknown/unknown.ex
@@ -0,0 +1,21 @@
+defmodule Reactor.Error.Unknown.UnknownError do
+  @moduledoc """
+  An error used to wrap unknown errors.
+  """
+
+  use Reactor.Error, fields: [:error], class: :unknown
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    """
+    # Unknown Error
+
+    An unknown error occurred.
+
+    ## `error`:
+
+    #{describe_error(error.error)}
+    """
+  end
+end
diff --git a/lib/reactor/error/utils.ex b/lib/reactor/error/utils.ex
new file mode 100644
index 0000000..cd57803
--- /dev/null
+++ b/lib/reactor/error/utils.ex
@@ -0,0 +1,53 @@
+defmodule Reactor.Error.Utils do
+  @moduledoc false
+
+  @doc "Attempt to describe an error"
+  @spec describe_error(any) :: String.t()
+  def describe_error(error) when is_exception(error), do: Exception.message(error)
+
+  def describe_error(error) when is_binary(error) do
+    if String.printable?(error) do
+      error
+    else
+      inspect_error(error)
+    end
+  end
+
+  def describe_error(error), do: inspect_error(error)
+
+  @doc "Helper function to provide suggestions in error messages"
+  @spec did_you_mean?(any, Enumerable.t(any)) :: nil | String.t()
+  def did_you_mean?(requested, possible) do
+    best_match =
+      possible
+      |> Enum.map(&inspect/1)
+      |> Enum.max_by(
+        &String.jaro_distance(&1, inspect(requested)),
+        &>=/2,
+        fn ->
+          nil
+        end
+      )
+
+    if best_match do
+      "Did you mean `#{inspect(best_match)}`?"
+    end
+  end
+
+  defp inspect_error(error) do
+    inspected =
+      error
+      |> inspect()
+      |> String.trim()
+
+    if inspected =~ ~r/[\r\n\v]/ do
+      """
+      ```
+      #{inspected}
+      ```
+      """
+    else
+      "`#{inspected}`"
+    end
+  end
+end
diff --git a/lib/reactor/error/validation.ex b/lib/reactor/error/validation.ex
new file mode 100644
index 0000000..925062a
--- /dev/null
+++ b/lib/reactor/error/validation.ex
@@ -0,0 +1,14 @@
+defmodule Reactor.Error.Validation do
+  @moduledoc """
+  The [Splode error class](e:splode:get-started-with-splode.html#error-classes)
+  for validation errors.
+  """
+
+  use Reactor.Error, fields: [:errors], class: :validation
+
+  @doc false
+  @impl true
+  def splode_message(%{errors: errors}) do
+    Splode.ErrorClass.error_messages(errors)
+  end
+end
diff --git a/lib/reactor/error/validation/missing_return_error.ex b/lib/reactor/error/validation/missing_return_error.ex
new file mode 100644
index 0000000..70469f7
--- /dev/null
+++ b/lib/reactor/error/validation/missing_return_error.ex
@@ -0,0 +1,22 @@
+defmodule Reactor.Error.Validation.MissingReturnError do
+  @moduledoc """
+  An error returned when a Reactor cannot be validated because of a missing
+  return value.
+  """
+
+  use Reactor.Error,
+    fields: [:reactor],
+    class: :validation
+
+  @doc false
+  @impl true
+  def splode_message(_error) do
+    """
+    # Missing Return Error
+
+    The Reactor does not have a named return value.
+
+    You can set one using `Reactor.Builder.return/2` or by setting the `d:Reactor.Dsl.reactor.return` DSL option.
+    """
+  end
+end
diff --git a/lib/reactor/error/validation/state_error.ex b/lib/reactor/error/validation/state_error.ex
new file mode 100644
index 0000000..5f13427
--- /dev/null
+++ b/lib/reactor/error/validation/state_error.ex
@@ -0,0 +1,39 @@
+defmodule Reactor.Error.Validation.StateError do
+  @moduledoc """
+  An error returned when a Reactor is in an unexpected state.
+  """
+
+  use Reactor.Error,
+    fields: [:reactor, :state, :expected],
+    class: :validation
+
+  @doc false
+  @impl true
+  def splode_message(error) do
+    """
+    # Reactor State Error
+
+    #{state_message(error)}
+    """
+  end
+
+  defp state_message(%{expected: [], state: state}),
+    do: "Reactor is in an invalid state: `#{inspect(state)}`"
+
+  defp state_message(%{expected: [expected], state: state}),
+    do: "Reactor is in an invalid state: `#{inspect(state)}`, expected: `#{inspect(expected)}`"
+
+  defp state_message(error) do
+    valid_states =
+      error.expected
+      |> Enum.map_join("\n", &"  * `#{inspect(&1)}`")
+
+    """
+    Reactor is in an invalid state: `#{inspect(error.state)}`
+
+    Expected states:
+
+    #{valid_states}
+    """
+  end
+end
diff --git a/lib/reactor/errors/retries_exceeded_error.ex b/lib/reactor/errors/retries_exceeded_error.ex
deleted file mode 100644
index 8fadcf2..0000000
--- a/lib/reactor/errors/retries_exceeded_error.ex
+++ /dev/null
@@ -1,27 +0,0 @@
-defmodule Reactor.Error.RetriesExceededError do
-  @moduledoc """
-  An error used when a step runs out of retry events and no other error is
-  thrown.
-  """
-  defexception [:step, :retry_count]
-
-  @doc false
-  @impl true
-  def exception(attrs), do: struct(__MODULE__, attrs)
-
-  @doc false
-  @impl true
-  def message(error) do
-    """
-    # Maximum number of retries exceeded executing step.
-
-    ## `retry_count`:
-
-    #{inspect(error.retry_count)}
-
-    ## `step`:
-
-    #{inspect(error.step)}
-    """
-  end
-end
diff --git a/lib/reactor/errors/transform_error.ex b/lib/reactor/errors/transform_error.ex
deleted file mode 100644
index 83833a1..0000000
--- a/lib/reactor/errors/transform_error.ex
+++ /dev/null
@@ -1,14 +0,0 @@
-defmodule Reactor.Error.TransformError do
-  @moduledoc """
-  An error which occurs when building and running transforms.
-  """
-  defexception input: nil, output: nil, message: nil
-
-  @doc false
-  @impl true
-  def exception(attrs), do: struct(__MODULE__, attrs)
-
-  @doc false
-  @impl true
-  def message(error), do: error.message
-end
diff --git a/lib/reactor/executor.ex b/lib/reactor/executor.ex
index 5ca465f..acfecb4 100644
--- a/lib/reactor/executor.ex
+++ b/lib/reactor/executor.ex
@@ -35,8 +35,15 @@ defmodule Reactor.Executor do
   3. When a step or compensation asks for a retry then the step is placed back
      in the graph to be run again next iteration.
   """
-  alias Reactor.Executor.ConcurrencyTracker
-  alias Reactor.{Executor, Planner, Step}
+  alias Reactor.{
+    Error.Internal.MissingReturnResultError,
+    Error.Validation.MissingReturnError,
+    Error.Validation.StateError,
+    Executor,
+    Executor.ConcurrencyTracker,
+    Planner,
+    Step
+  }
 
   @doc """
   Run a reactor.
@@ -51,7 +58,7 @@ defmodule Reactor.Executor do
   def run(reactor, inputs \\ %{}, context \\ %{}, options \\ [])
 
   def run(reactor, _inputs, _context, _options) when is_nil(reactor.return),
-    do: {:error, ArgumentError.exception("`reactor` has no return value")}
+    do: {:error, MissingReturnError.exception(reactor: reactor)}
 
   def run(reactor, inputs, context, options) when reactor.state in ~w[pending halted]a do
     with {:ok, context} <- Executor.Hooks.init(reactor, context),
@@ -60,8 +67,14 @@ defmodule Reactor.Executor do
     end
   end
 
-  def run(_reactor, _inputs, _context, _options),
-    do: {:error, ArgumentError.exception("`reactor` is not in `pending` or `halted` state")}
+  def run(reactor, _inputs, _context, _options),
+    do:
+      {:error,
+       StateError.exception(
+         reactor: reactor,
+         state: reactor.state,
+         expected: ~w[pending halted]a
+       )}
 
   defp execute(reactor, state) when state.max_iterations == 0 do
     {reactor, _status} = Executor.Async.collect_remaining_tasks_for_shutdown(reactor, state)
@@ -219,7 +232,8 @@ defmodule Reactor.Executor do
   end
 
   defp handle_undo(reactor, state, []) do
-    Executor.Hooks.error(reactor, state.errors, reactor.context)
+    error = Reactor.Error.to_class(state.errors)
+    Executor.Hooks.error(reactor, error, reactor.context)
   end
 
   defp handle_undo(reactor, state, [{step, value} | tail]) do
@@ -234,8 +248,11 @@ defmodule Reactor.Executor do
          {:ok, value} <- Map.fetch(reactor.intermediate_results, reactor.return) do
       {:ok, value}
     else
-      :error -> {:error, "Unable to find result for `#{inspect(reactor.return)}` step"}
-      n when is_integer(n) -> {:continue, reactor}
+      :error ->
+        {:error, MissingReturnResultError.exception(reactor: reactor)}
+
+      n when is_integer(n) ->
+        {:continue, reactor}
     end
   end
 
diff --git a/lib/reactor/executor/async.ex b/lib/reactor/executor/async.ex
index 08dd763..35616c5 100644
--- a/lib/reactor/executor/async.ex
+++ b/lib/reactor/executor/async.ex
@@ -3,8 +3,9 @@ defmodule Reactor.Executor.Async do
   Handle the asynchronous execution of a batch of steps, along with any
   mutations to the reactor or execution state.
   """
+  alias Reactor.Error.Invalid.RetriesExceededError, as: RetriesExceededError
   alias Reactor.Executor.ConcurrencyTracker
-  alias Reactor.{Error, Executor, Step}
+  alias Reactor.{Executor, Step}
   require Logger
 
   @doc """
@@ -276,7 +277,7 @@ defmodule Reactor.Executor.Async do
           reason
 
         {step, :retry} ->
-          Error.RetriesExceededError.exception(
+          RetriesExceededError.exception(
             step: step,
             retry_count: Map.get(state.retries, step.ref)
           )
diff --git a/lib/reactor/executor/step_runner.ex b/lib/reactor/executor/step_runner.ex
index 1427eca..aaaeb65 100644
--- a/lib/reactor/executor/step_runner.ex
+++ b/lib/reactor/executor/step_runner.ex
@@ -2,7 +2,20 @@ defmodule Reactor.Executor.StepRunner do
   @moduledoc """
   Run an individual step, including compensation if possible.
   """
-  alias Reactor.{Executor.ConcurrencyTracker, Executor.Hooks, Executor.State, Step}
+  alias Reactor.{
+    Error.Invalid.ArgumentSubpathError,
+    Error.Invalid.CompensateStepError,
+    Error.Invalid.MissingInputError,
+    Error.Invalid.MissingResultError,
+    Error.Invalid.RunStepError,
+    Error.Invalid.UndoRetriesExceededError,
+    Error.Invalid.UndoStepError,
+    Executor.ConcurrencyTracker,
+    Executor.Hooks,
+    Executor.State,
+    Step
+  }
+
   import Reactor.Utils
   import Reactor.Argument, only: :macros
   require Logger
@@ -67,11 +80,11 @@ defmodule Reactor.Executor.StepRunner do
 
   defp do_undo(reactor, _value, step, _arguments, context, undo_count)
        when undo_count == @max_undo_count do
-    reason = "`undo/4` retried #{@max_undo_count} times on step `#{inspect(step.name)}`."
+    error = UndoRetriesExceededError.exception(step: step.name, retry_count: undo_count)
 
-    Hooks.event(reactor, {:undo_error, reason}, step, context)
+    Hooks.event(reactor, {:undo_error, error}, step, context)
 
-    {:error, reason}
+    {:error, error}
   end
 
   defp do_undo(reactor, value, step, arguments, context, undo_count) do
@@ -90,8 +103,9 @@ defmodule Reactor.Executor.StepRunner do
         do_undo(reactor, value, step, arguments, context, undo_count + 1)
 
       {:error, reason} ->
-        Hooks.event(reactor, {:undo_error, reason}, step, context)
-        {:error, reason}
+        error = UndoStepError.exception(step: step.name, error: reason)
+        Hooks.event(reactor, {:undo_error, error}, step, context)
+        {:error, error}
     end
   end
 
@@ -103,9 +117,10 @@ defmodule Reactor.Executor.StepRunner do
     |> handle_run_result(reactor, step, arguments, context)
   rescue
     reason ->
-      Hooks.event(reactor, {:run_error, reason}, step, context)
+      error = RunStepError.exception(step: step.name, error: reason)
+      Hooks.event(reactor, {:run_error, error}, step, context)
 
-      maybe_compensate(reactor, step, reason, arguments, context)
+      maybe_compensate(reactor, step, error, arguments, context)
   end
 
   defp handle_run_result({:ok, value}, reactor, step, _arguments, context) do
@@ -134,9 +149,10 @@ defmodule Reactor.Executor.StepRunner do
   end
 
   defp handle_run_result({:error, reason}, reactor, step, arguments, context) do
-    Hooks.event(reactor, {:run_error, reason}, step, context)
+    error = RunStepError.exception(step: step.name, error: reason)
+    Hooks.event(reactor, {:run_error, error}, step, context)
 
-    maybe_compensate(reactor, step, reason, arguments, context)
+    maybe_compensate(reactor, step, error, arguments, context)
   end
 
   defp handle_run_result({:halt, value}, reactor, step, _arguments, context) do
@@ -145,30 +161,38 @@ defmodule Reactor.Executor.StepRunner do
     {:halt, value}
   end
 
-  defp maybe_compensate(reactor, step, reason, arguments, context) do
+  defp maybe_compensate(reactor, step, error, arguments, context) do
     if Step.can?(step, :compensate) do
-      compensate(reactor, step, reason, arguments, context)
+      compensate(reactor, step, error, arguments, context)
     else
-      {:error, reason}
+      {:error, error}
     end
   end
 
-  defp compensate(reactor, step, reason, arguments, context) do
-    Hooks.event(reactor, {:compensate_start, reason}, step, context)
+  defp compensate(reactor, step, error, arguments, context) do
+    Hooks.event(reactor, {:compensate_start, error}, step, context)
 
     step
-    |> Step.compensate(reason, arguments, context)
-    |> handle_compensate_result(reactor, step, context, reason)
+    |> Step.compensate(error.error, arguments, context)
+    |> handle_compensate_result(reactor, step, context, error)
   rescue
     error ->
-      Hooks.event(reactor, {:compensate_error, reason}, step, context)
+      error =
+        CompensateStepError.exception(
+          reactor: reactor,
+          step: step,
+          error: error,
+          stacktrace: __STACKTRACE__
+        )
+
+      Hooks.event(reactor, {:compensate_error, error}, step, context)
 
       Logger.error(fn ->
         "Warning: step `#{inspect(step.name)}` `compensate/4` raised an error:\n" <>
           Exception.format(:error, error, __STACKTRACE__)
       end)
 
-      {:error, reason}
+      {:error, error}
   end
 
   defp handle_compensate_result({:continue, value}, reactor, step, context, _) do
@@ -190,15 +214,17 @@ defmodule Reactor.Executor.StepRunner do
   end
 
   defp handle_compensate_result({:error, reason}, reactor, step, context, _) do
-    Hooks.event(reactor, {:compensate_error, reason}, step, context)
+    error = CompensateStepError.exception(reactor: reactor, step: step, error: reason)
 
-    {:error, reason}
+    Hooks.event(reactor, {:compensate_error, error}, step, context)
+
+    {:error, error}
   end
 
-  defp handle_compensate_result(:ok, reactor, step, context, reason) do
+  defp handle_compensate_result(:ok, reactor, step, context, error) do
     Hooks.event(reactor, :compensate_complete, step, context)
 
-    {:error, reason}
+    {:error, error}
   end
 
   defp get_step_arguments(reactor, step) do
@@ -208,7 +234,7 @@ defmodule Reactor.Executor.StepRunner do
 
       argument, arguments ->
         with {:ok, value} <- fetch_argument(reactor, step, argument),
-             {:ok, value} <- subpath_argument(value, argument) do
+             {:ok, value} <- subpath_argument(value, step, argument) do
           {:ok, Map.put(arguments, argument.name, value)}
         end
     end)
@@ -216,14 +242,13 @@ defmodule Reactor.Executor.StepRunner do
 
   defp fetch_argument(reactor, step, argument) when is_from_input(argument) do
     with :error <- Map.fetch(reactor.context.private.inputs, argument.source.name) do
-      {:error,
-       "Step `#{inspect(step.name)}` argument `#{inspect(argument.name)}` relies on missing input `#{argument.source.name}`"}
+      {:error, MissingInputError.exception(reactor: reactor, step: step, argument: argument)}
     end
   end
 
   defp fetch_argument(reactor, step, argument) when is_from_result(argument) do
     with :error <- Map.fetch(reactor.intermediate_results, argument.source.name) do
-      {:error, "Step `#{inspect(step.name)}` argument `#{inspect(argument.name)}` is missing"}
+      {:error, MissingResultError.exception(reactor: reactor, step: step, argument: argument)}
     end
   end
 
@@ -231,44 +256,125 @@ defmodule Reactor.Executor.StepRunner do
     {:ok, argument.source.value}
   end
 
-  defp subpath_argument(value, argument) when has_sub_path(argument),
-    do: perform_argument_subpath(value, argument.name, argument.source.sub_path, [])
+  defp subpath_argument(value, step, argument) when has_sub_path(argument),
+    do: perform_argument_subpath(value, step, argument, argument.source.sub_path, [], value)
 
-  defp subpath_argument(value, _argument), do: {:ok, value}
+  defp subpath_argument(value, _step, _argument), do: {:ok, value}
 
-  defp perform_argument_subpath(value, _, [], _), do: {:ok, value}
+  defp perform_argument_subpath(
+         value,
+         step,
+         argument,
+         remaining_path,
+         done_path,
+         intermediate_value
+       )
 
-  defp perform_argument_subpath(value, name, remaining, done) when is_struct(value),
-    do: value |> Map.from_struct() |> perform_argument_subpath(name, remaining, done)
+  defp perform_argument_subpath(_value, _step, _argument, [], _, result), do: {:ok, result}
 
-  defp perform_argument_subpath(value, name, [head | tail], []) do
-    case access_fetch_with_rescue(value, head) do
-      {:ok, value} ->
-        perform_argument_subpath(value, name, tail, [head])
+  defp perform_argument_subpath(
+         value,
+         step,
+         argument,
+         [key | remaining_path],
+         done_path,
+         intermediate_value
+       )
+       when is_map(intermediate_value) do
+    case Map.fetch(intermediate_value, key) do
+      {:ok, intermediate_value} ->
+        perform_argument_subpath(
+          value,
+          step,
+          argument,
+          remaining_path,
+          [key | done_path],
+          intermediate_value
+        )
 
       :error ->
+        type = if is_struct(intermediate_value), do: "struct", else: "map"
+
         {:error,
-         "Unable to resolve subpath for argument `#{inspect(name)}` at key `[#{inspect(head)}]`"}
+         ArgumentSubpathError.exception(
+           step: step,
+           argument: argument,
+           culprit: intermediate_value,
+           culprit_path: done_path,
+           culprit_key: key,
+           value: value,
+           message:
+             "key `#{inspect(key)}` not present in #{type} at path `#{inspect(done_path)}`."
+         )}
     end
   end
 
-  defp perform_argument_subpath(value, name, [head | tail], done) do
-    case access_fetch_with_rescue(value, head) do
-      {:ok, value} ->
-        perform_argument_subpath(value, name, tail, [head])
+  defp perform_argument_subpath(
+         value,
+         step,
+         argument,
+         [key | remaining_path],
+         done_path,
+         intermediate_value
+       )
+       when is_list(intermediate_value) do
+    if Keyword.keyword?(intermediate_value) do
+      case Keyword.fetch(intermediate_value, key) do
+        {:ok, intermediate_value} ->
+          perform_argument_subpath(
+            value,
+            step,
+            argument,
+            remaining_path,
+            [key | done_path],
+            intermediate_value
+          )
 
-      :error ->
-        path = Enum.reverse([head | done])
-
-        {:error,
-         "Unable to resolve subpath for argument `#{inspect(name)}` at key `#{inspect(path)}`"}
+        :error ->
+          {:error,
+           ArgumentSubpathError.exception(
+             step: step,
+             argument: argument,
+             culprit: intermediate_value,
+             culprit_path: done_path,
+             culprit_key: key,
+             value: value,
+             message:
+               "key `#{inspect(key)}` not present in keyword list at path `#{inspect(done_path)}`."
+           )}
+      end
+    else
+      {:error,
+       ArgumentSubpathError.exception(
+         step: step,
+         argument: argument,
+         value: value,
+         culprit: intermediate_value,
+         culprit_key: List.first(done_path),
+         culprit_path: done_path,
+         message: "list at path `#{inspect(done_path)}` is not a keyword list."
+       )}
     end
   end
 
-  defp access_fetch_with_rescue(container, key) do
-    Access.fetch(container, key)
-  rescue
-    FunctionClauseError -> :error
+  defp perform_argument_subpath(
+         value,
+         step,
+         argument,
+         _remaining_path,
+         done_path,
+         intermediate_value
+       ) do
+    {:error,
+     ArgumentSubpathError.exception(
+       step: step,
+       argument: argument,
+       value: value,
+       culprit: intermediate_value,
+       culprit_path: done_path,
+       culprit_key: List.first(done_path),
+       message: "value is neither a map or keyword list."
+     )}
   end
 
   defp build_context(reactor, state, step, concurrency_key) do
diff --git a/lib/reactor/executor/sync.ex b/lib/reactor/executor/sync.ex
index 77a1f73..f94bc6d 100644
--- a/lib/reactor/executor/sync.ex
+++ b/lib/reactor/executor/sync.ex
@@ -4,7 +4,8 @@ defmodule Reactor.Executor.Sync do
   the reactor or execution state.
   """
 
-  alias Reactor.{Error, Executor, Step}
+  alias Reactor.Error.Invalid.RetriesExceededError, as: RetriesExceededError
+  alias Reactor.{Executor, Step}
 
   @doc """
   Try and run a step synchronously.
@@ -22,7 +23,7 @@ defmodule Reactor.Executor.Sync do
           reactor = drop_from_plan(reactor, step)
 
           error =
-            Error.RetriesExceededError.exception(
+            RetriesExceededError.exception(
               step: step,
               retry_count: Map.get(state.retries, step.ref)
             )
diff --git a/lib/reactor/planner.ex b/lib/reactor/planner.ex
index 827bf2f..dd51df7 100644
--- a/lib/reactor/planner.ex
+++ b/lib/reactor/planner.ex
@@ -6,7 +6,7 @@ defmodule Reactor.Planner do
   between them representing their dependencies (arguments).
   """
 
-  alias Reactor.{Error.PlanError, Step}
+  alias Reactor.{Error.Internal.PlanError, Step}
   import Reactor, only: :macros
   import Reactor.Argument, only: :macros
   import Reactor.Utils
diff --git a/lib/reactor/step.ex b/lib/reactor/step.ex
index a9cb62a..b8a91b5 100644
--- a/lib/reactor/step.ex
+++ b/lib/reactor/step.ex
@@ -118,7 +118,7 @@ defmodule Reactor.Step do
     - `:retry` or `{:retry, reason}` if you would like the reactor to attempt to
       re-run the step. You can optionally supply an error reason which will be
       used in the event that the step runs out of retries, otherwise a
-      `Reactor.Error.RetriesExceededError` will be used.
+      `Reactor.Error.Invalid.RetriesExceededError` will be used.
     - `{:error, reason}` if compensation was unsuccessful.
   """
   @callback compensate(
diff --git a/lib/reactor/step/around.ex b/lib/reactor/step/around.ex
index 2bbc7c8..2769c70 100644
--- a/lib/reactor/step/around.ex
+++ b/lib/reactor/step/around.ex
@@ -107,7 +107,6 @@ defmodule Reactor.Step.Around do
          {:ok, result} <- fun.(arguments, context, steps, &__MODULE__.around(&1, &2, &3, options)) do
       {:ok, result}
     else
-      :error -> {:error, "Missing `fun` option."}
       {:error, reason} -> {:error, reason}
     end
   end
@@ -154,9 +153,12 @@ defmodule Reactor.Step.Around do
            end}
         end)
 
-      _ ->
+      {:ok, _} ->
         {:error,
          argument_error(:options, "Expected `fun` option to be a 4 arity function", options)}
+
+      :error ->
+        {:error, argument_error(:options, "The required option `fun` is not present", options)}
     end
   end
 
@@ -164,7 +166,12 @@ defmodule Reactor.Step.Around do
     if Code.ensure_loaded?(m) && function_exported?(m, f, arity) do
       callback.()
     else
-      {:error, "Expected `#{inspect(m)}.#{f}/#{arity}` to be exported."}
+      {:error,
+       argument_error(
+         :mfa,
+         "Expected `#{inspect(m)}.#{f}/#{arity}` to be exported.",
+         {m, f, arity}
+       )}
     end
   end
 
diff --git a/lib/reactor/step/compose.ex b/lib/reactor/step/compose.ex
index 1487eb5..d4d8160 100644
--- a/lib/reactor/step/compose.ex
+++ b/lib/reactor/step/compose.ex
@@ -12,7 +12,7 @@ defmodule Reactor.Step.Compose do
   """
 
   use Reactor.Step
-  alias Reactor.{Argument, Builder, Error.ComposeError, Info, Step}
+  alias Reactor.{Argument, Builder, Error.Internal.ComposeError, Info, Step}
   import Reactor, only: :macros
   import Reactor.Argument, only: :macros
   import Reactor.Utils
diff --git a/lib/reactor/step/transform.ex b/lib/reactor/step/transform.ex
index f428e5b..ffcb802 100644
--- a/lib/reactor/step/transform.ex
+++ b/lib/reactor/step/transform.ex
@@ -6,16 +6,24 @@ defmodule Reactor.Step.Transform do
   semantics.
   """
 
-  alias Reactor.Step
+  alias Reactor.{Error.Invalid.MissingArgumentError, Error.Invalid.TransformError, Step}
   use Step
 
   @doc false
   @impl true
   @spec run(Reactor.inputs(), Reactor.context(), keyword) :: {:ok | :error, any}
-  def run(arguments, _context, options) do
+  def run(arguments, context, options) do
     case Map.fetch(arguments, :value) do
-      {:ok, value} -> do_transform(value, options)
-      :error -> {:error, ArgumentError.exception("The `value` argument is missing")}
+      {:ok, value} ->
+        do_transform(value, options)
+
+      :error ->
+        {:error,
+         MissingArgumentError.exception(
+           step: context.current_step,
+           argument: :value,
+           arguments: arguments
+         )}
     end
   end
 
@@ -31,6 +39,6 @@ defmodule Reactor.Step.Transform do
         {:ok, apply(m, f, [value | a])}
     end
   rescue
-    error -> {:error, error}
+    error -> {:error, TransformError.exception(input: value, error: error)}
   end
 end
diff --git a/lib/reactor/step/transform_all.ex b/lib/reactor/step/transform_all.ex
index e2e1e96..40ed420 100644
--- a/lib/reactor/step/transform_all.ex
+++ b/lib/reactor/step/transform_all.ex
@@ -7,7 +7,7 @@ defmodule Reactor.Step.TransformAll do
   """
 
   use Reactor.Step
-  alias Reactor.{Error.TransformError, Step.Transform}
+  alias Reactor.{Error.Invalid.TransformError, Step.Transform}
 
   @doc false
   @impl true
@@ -22,11 +22,11 @@ defmodule Reactor.Step.TransformAll do
          TransformError.exception(
            input: arguments,
            output: result,
-           message: "Step transformers must return a map to use as replacement arguments."
+           error: "Step transformers must return a map to use as replacement arguments."
          )}
 
       {:error, reason} ->
-        {:error, reason}
+        {:error, TransformError.exception(input: arguments, error: reason)}
     end
   end
 end
diff --git a/mix.exs b/mix.exs
index fef4190..966fbbd 100644
--- a/mix.exs
+++ b/mix.exs
@@ -90,6 +90,7 @@ defmodule Reactor.MixProject do
   defp deps do
     [
       {:spark, "~> 2.0"},
+      {:splode, "~> 0.1.0"},
       {:libgraph, "~> 0.16"},
       {:telemetry, "~> 1.2"},
 
diff --git a/mix.lock b/mix.lock
index 96b7a1a..d980495 100644
--- a/mix.lock
+++ b/mix.lock
@@ -23,6 +23,7 @@
   "sobelow": {:hex, :sobelow, "0.13.0", "218afe9075904793f5c64b8837cc356e493d88fddde126a463839351870b8d1e", [:mix], [{:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "cd6e9026b85fc35d7529da14f95e85a078d9dd1907a9097b3ba6ac7ebbe34a0d"},
   "sourceror": {:hex, :sourceror, "1.0.2", "c5e86fdc14881f797749d1fe5df017ca66727a8146e7ee3e736605a3df78f3e6", [:mix], [], "hexpm", "832335e87d0913658f129d58b2a7dc0490ddd4487b02de6d85bca0169ec2bd79"},
   "spark": {:hex, :spark, "2.1.0", "71b27a34c4eb6e9df958237ecb5df1f738465c05a190f3024fed6c305adff903", [:mix], [{:jason, "~> 1.4", [hex: :jason, repo: "hexpm", optional: false]}, {:sourceror, "~> 1.0", [hex: :sourceror, repo: "hexpm", optional: false]}], "hexpm", "6719170085ec23c9ecd48c1a92cef0c596acec7f77b2265a4b32e92a4c6a7daa"},
+  "splode": {:hex, :splode, "0.1.1", "1e3290c2d11f95bd3c3e6cf44cd33261ce76e2429a6d367f7896fd84698ca29f", [:mix], [], "hexpm", "7bf9dd0ade39c5074434776d4e10e4d71e88707a39bee9e809438a73bc0e92a9"},
   "telemetry": {:hex, :telemetry, "1.2.1", "68fdfe8d8f05a8428483a97d7aab2f268aaff24b49e0f599faa091f1d4e7f61c", [:rebar3], [], "hexpm", "dad9ce9d8effc621708f99eac538ef1cbe05d6a874dd741de2e689c47feafed5"},
   "yamerl": {:hex, :yamerl, "0.10.0", "4ff81fee2f1f6a46f1700c0d880b24d193ddb74bd14ef42cb0bcf46e81ef2f8e", [:rebar3], [], "hexpm", "346adb2963f1051dc837a2364e4acf6eb7d80097c0f53cbdc3046ec8ec4b4e6e"},
   "yaml_elixir": {:hex, :yaml_elixir, "2.9.0", "9a256da867b37b8d2c1ffd5d9de373a4fda77a32a45b452f1708508ba7bbcb53", [:mix], [{:yamerl, "~> 0.10", [hex: :yamerl, repo: "hexpm", optional: false]}], "hexpm", "0cb0e7d4c56f5e99a6253ed1a670ed0e39c13fc45a6da054033928607ac08dfc"},
diff --git a/test/reactor/builder/compose_test.exs b/test/reactor/builder/compose_test.exs
index 29bc36f..fb53b88 100644
--- a/test/reactor/builder/compose_test.exs
+++ b/test/reactor/builder/compose_test.exs
@@ -1,6 +1,16 @@
 defmodule Reactor.Builder.ComposeTest do
   use ExUnit.Case, async: true
-  alias Reactor.{Argument, Builder, Builder.Compose, Error.ComposeError, Planner, Step, Template}
+
+  alias Reactor.{
+    Argument,
+    Builder,
+    Builder.Compose,
+    Error.Internal.ComposeError,
+    Planner,
+    Step,
+    Template
+  }
+
   require Reactor.Argument
 
   describe "compose/4" do
diff --git a/test/reactor/executor/async_test.exs b/test/reactor/executor/async_test.exs
index 0b2c802..5b6507f 100644
--- a/test/reactor/executor/async_test.exs
+++ b/test/reactor/executor/async_test.exs
@@ -1,5 +1,6 @@
 defmodule Reactor.Executor.AsyncTest do
-  alias Reactor.{Error, Executor}
+  alias Reactor.Error.Invalid.RetriesExceededError, as: RetriesExceededError
+  alias Reactor.Executor
   import Reactor.Executor.Async
   use ExUnit.Case, async: true
 
@@ -224,7 +225,7 @@ defmodule Reactor.Executor.AsyncTest do
       task = Task.Supervisor.async_nolink(supervisor, fn -> :retry end)
       state = %{state | current_tasks: %{task => undoable}, retries: %{undoable.ref => 100}}
 
-      assert {:undo, _reactor, %{errors: [%Error.RetriesExceededError{}]}} =
+      assert {:undo, _reactor, %{errors: [%RetriesExceededError{}]}} =
                handle_completed_steps(reactor, state)
     end
   end
diff --git a/test/reactor/executor/event_test.exs b/test/reactor/executor/event_test.exs
index 1edb1d8..13de1cd 100644
--- a/test/reactor/executor/event_test.exs
+++ b/test/reactor/executor/event_test.exs
@@ -1,6 +1,11 @@
 defmodule Reactor.Executor.EventTest do
   use ExUnit.Case, async: true
 
+  alias Reactor.Error.Invalid.CompensateStepError
+  alias Reactor.Error.Invalid.RunStepError
+  alias Reactor.Error.Invalid.UndoRetriesExceededError
+  alias Reactor.Error.Invalid.UndoStepError
+
   defmodule EventMiddleware do
     use Reactor.Middleware
 
@@ -58,7 +63,7 @@ defmodule Reactor.Executor.EventTest do
     test "fail step" do
       assert [
                {:run_start, _},
-               {:run_error, :marty}
+               {:run_error, %RunStepError{error: :marty}}
              ] =
                run(StepReactor, %{step_result: {:error, :marty}}, async?: false)
     end
@@ -96,8 +101,8 @@ defmodule Reactor.Executor.EventTest do
     test "successful compensation events" do
       assert [
                {:run_start, _},
-               {:run_error, :fail},
-               {:compensate_start, :fail},
+               {:run_error, %RunStepError{error: :fail}},
+               {:compensate_start, %RunStepError{error: :fail}},
                :compensate_complete
              ] =
                run(CompensateReactor, %{compensation_result: :ok}, async?: false)
@@ -106,12 +111,12 @@ defmodule Reactor.Executor.EventTest do
     test "compensation retries" do
       assert [
                {:run_start, _},
-               {:run_error, :fail},
-               {:compensate_start, :fail},
+               {:run_error, %RunStepError{error: :fail}},
+               {:compensate_start, %RunStepError{error: :fail}},
                :compensate_retry,
                {:run_start, _},
-               {:run_error, :fail},
-               {:compensate_start, :fail},
+               {:run_error, %RunStepError{error: :fail}},
+               {:compensate_start, %RunStepError{error: :fail}},
                :compensate_retry
              ] = run(CompensateReactor, %{compensation_result: :retry}, async?: false)
     end
@@ -119,9 +124,9 @@ defmodule Reactor.Executor.EventTest do
     test "compensation failure" do
       assert [
                {:run_start, _},
-               {:run_error, :fail},
-               {:compensate_start, :fail},
-               {:compensate_error, :cant_compensate}
+               {:run_error, %RunStepError{error: :fail}},
+               {:compensate_start, %RunStepError{error: :fail}},
+               {:compensate_error, %CompensateStepError{error: :cant_compensate}}
              ] =
                run(CompensateReactor, %{compensation_result: {:error, :cant_compensate}},
                  async?: false
@@ -131,8 +136,8 @@ defmodule Reactor.Executor.EventTest do
     test "compensation complete" do
       assert [
                {:run_start, _},
-               {:run_error, :fail},
-               {:compensate_start, :fail},
+               {:run_error, %RunStepError{error: :fail}},
+               {:compensate_start, %RunStepError{error: :fail}},
                :compensate_complete
              ] =
                run(CompensateReactor, %{compensation_result: :ok}, async?: false)
@@ -141,8 +146,8 @@ defmodule Reactor.Executor.EventTest do
     test "compensation continue" do
       assert [
                {:run_start, _},
-               {:run_error, :fail},
-               {:compensate_start, :fail},
+               {:run_error, %RunStepError{error: :fail}},
+               {:compensate_start, %RunStepError{error: :fail}},
                {:compensate_continue, :all_is_well}
              ] =
                run(CompensateReactor, %{compensation_result: {:continue, :all_is_well}},
@@ -181,7 +186,7 @@ defmodule Reactor.Executor.EventTest do
                {:run_start, _},
                {:run_complete, :marty},
                {:run_start, _},
-               {:run_error, :doc_brown},
+               {:run_error, %RunStepError{error: :doc_brown}},
                :undo_start,
                :undo_complete
              ] =
@@ -193,14 +198,14 @@ defmodule Reactor.Executor.EventTest do
                {:run_start, _},
                {:run_complete, :marty},
                {:run_start, _},
-               {:run_error, :doc_brown},
+               {:run_error, %RunStepError{error: :doc_brown}},
                :undo_start,
                :undo_retry,
                :undo_retry,
                :undo_retry,
                :undo_retry,
                :undo_retry,
-               {:undo_error, "`undo/4` retried 5 times on step `:undo_step`."}
+               {:undo_error, %UndoRetriesExceededError{}}
              ] =
                run(UndoReactor, %{undo_result: :retry}, async?: false)
     end
@@ -210,14 +215,14 @@ defmodule Reactor.Executor.EventTest do
                {:run_start, %{undo_result: {:retry, :einstein}}},
                {:run_complete, :marty},
                {:run_start, %{}},
-               {:run_error, :doc_brown},
+               {:run_error, %RunStepError{error: :doc_brown}},
                :undo_start,
                {:undo_retry, :einstein},
                {:undo_retry, :einstein},
                {:undo_retry, :einstein},
                {:undo_retry, :einstein},
                {:undo_retry, :einstein},
-               {:undo_error, "`undo/4` retried 5 times on step `:undo_step`."}
+               {:undo_error, %UndoRetriesExceededError{}}
              ] =
                run(UndoReactor, %{undo_result: {:retry, :einstein}}, async?: false)
     end
@@ -227,9 +232,9 @@ defmodule Reactor.Executor.EventTest do
                {:run_start, _},
                {:run_complete, :marty},
                {:run_start, _},
-               {:run_error, :doc_brown},
+               {:run_error, %RunStepError{error: :doc_brown}},
                :undo_start,
-               {:undo_error, :einstein}
+               {:undo_error, %UndoStepError{error: :einstein}}
              ] = run(UndoReactor, %{undo_result: {:error, :einstein}}, async?: false)
     end
   end
diff --git a/test/reactor/executor/hooks_test.exs b/test/reactor/executor/hooks_test.exs
index 3e742c7..79a2870 100644
--- a/test/reactor/executor/hooks_test.exs
+++ b/test/reactor/executor/hooks_test.exs
@@ -1,7 +1,7 @@
 defmodule Reactor.Executor.HooksTest do
   @moduledoc false
   use ExUnit.Case, async: true
-  alias Reactor.Builder
+  alias Reactor.{Builder, Error.Invalid.RunStepError}
 
   describe "init" do
     defmodule ReturnContextReactor do
@@ -84,10 +84,8 @@ defmodule Reactor.Executor.HooksTest do
         @moduledoc false
         @behaviour Reactor.Middleware
 
-        def error(errors, _context) do
-          [error] = List.wrap(errors)
-          assert is_exception(error, RuntimeError)
-          assert Exception.message(error) == "hell"
+        def error(error, _context) do
+          assert Exception.message(error) =~ "hell"
 
           {:error, :wat}
         end
@@ -116,7 +114,7 @@ defmodule Reactor.Executor.HooksTest do
         ErrorReactor.reactor()
         |> Builder.add_middleware!(ErrorContextMiddleware)
 
-      assert {:error, [%RuntimeError{message: "hell"}]} =
+      assert {:error, %{errors: [%RunStepError{error: %RuntimeError{message: "hell"}}]}} =
                Reactor.run(reactor, %{}, %{is_context?: true})
     end
   end
diff --git a/test/reactor/executor/step_runner_test.exs b/test/reactor/executor/step_runner_test.exs
index 168c04a..dc64e2a 100644
--- a/test/reactor/executor/step_runner_test.exs
+++ b/test/reactor/executor/step_runner_test.exs
@@ -1,7 +1,18 @@
 defmodule Reactor.Executor.StepRunnerTest do
   @moduledoc false
   use ExUnit.Case, async: true
-  alias Reactor.{Argument, Builder, Executor.State, Template}
+
+  alias Reactor.{
+    Argument,
+    Builder,
+    Error.Invalid.ArgumentSubpathError,
+    Error.Invalid.MissingResultError,
+    Error.Invalid.RunStepError,
+    Error.Invalid.UndoRetriesExceededError,
+    Executor.State,
+    Template
+  }
+
   import Reactor.Executor.StepRunner
   use Mimic
 
@@ -21,8 +32,8 @@ defmodule Reactor.Executor.StepRunnerTest do
       {:ok, reactor} = Builder.add_step(reactor, :marty, Example.Step.Doable, [argument])
       step = reactor.steps |> hd()
 
-      assert {:error, reason} = run(reactor, state, step, nil)
-      assert reason =~ "argument `:current_year` is missing"
+      assert {:error, %MissingResultError{argument: %{source: %{name: :time_circuits}}}} =
+               run(reactor, state, step, nil)
     end
 
     test "when the required argument cannot be subpathed, it returns an error", %{
@@ -40,8 +51,8 @@ defmodule Reactor.Executor.StepRunnerTest do
       step = reactor.steps |> hd()
       reactor = %{reactor | intermediate_results: %{time_circuits: 1985}}
 
-      assert {:error, reason} = run(reactor, state, step, nil)
-      assert reason == "Unable to resolve subpath for argument `:current_year` at key `[:year]`"
+      assert {:error, %ArgumentSubpathError{argument: %{name: :current_year}}} =
+               run(reactor, state, step, nil)
     end
 
     test "when the required argument can be subpathed, it calls the step with the correct arguments",
@@ -145,7 +156,7 @@ defmodule Reactor.Executor.StepRunnerTest do
         {:error, :doc}
       end)
 
-      assert {:error, :doc} = run(reactor, state, step, nil)
+      assert {:error, %RunStepError{error: :doc}} = run(reactor, state, step, nil)
     end
 
     test "when a step raises an error it returns an error tuple", %{
@@ -161,8 +172,7 @@ defmodule Reactor.Executor.StepRunnerTest do
       end)
 
       assert {:error, error} = run(reactor, state, step, nil)
-      assert is_struct(error, RuntimeError)
-      assert Exception.message(error) == "Not enough plutonium!"
+      assert Exception.message(error) =~ "Not enough plutonium!"
     end
 
     test "when a step returns an error and can be compensated and the compensation says it can continue it returns an ok tuple",
@@ -186,7 +196,7 @@ defmodule Reactor.Executor.StepRunnerTest do
       |> stub(:run, fn _, _, _ -> {:error, :doc} end)
       |> stub(:compensate, fn :doc, _, _, _ -> :ok end)
 
-      assert {:error, :doc} = run(reactor, state, step, nil)
+      assert {:error, %RunStepError{error: :doc}} = run(reactor, state, step, nil)
     end
   end
 
@@ -241,8 +251,8 @@ defmodule Reactor.Executor.StepRunnerTest do
       Example.Step.Undoable
       |> stub(:undo, fn _, _, _, _ -> :retry end)
 
-      assert {:error, message} = undo(reactor, state, step, :marty, nil)
-      assert message =~ "retried 5 times"
+      assert {:error, %UndoRetriesExceededError{step: :marty}} =
+               undo(reactor, state, step, :marty, nil)
     end
   end
 end
diff --git a/test/reactor/executor/sync_test.exs b/test/reactor/executor/sync_test.exs
index c0a26b2..1bfffea 100644
--- a/test/reactor/executor/sync_test.exs
+++ b/test/reactor/executor/sync_test.exs
@@ -1,5 +1,6 @@
 defmodule Reactor.Executor.SyncTest do
-  alias Reactor.{Error, Executor}
+  alias Reactor.Error.Invalid.RetriesExceededError, as: RetriesExceededError
+  alias Reactor.Executor
   import Reactor.Executor.Sync
   use ExUnit.Case, async: true
   use Mimic
@@ -77,7 +78,9 @@ defmodule Reactor.Executor.SyncTest do
       |> stub(:run, fn _, _, _ -> :retry end)
 
       state = %{state | retries: Map.put(state.retries, step.ref, 100)}
-      assert {:undo, _, %{errors: [%Error.RetriesExceededError{}]}} = run(reactor, state, step)
+
+      assert {:undo, _, %{errors: [%RetriesExceededError{}]}} =
+               run(reactor, state, step)
     end
 
     test "when the step is successful it tells the reactor to recurse", %{
diff --git a/test/reactor/executor_test.exs b/test/reactor/executor_test.exs
index c0f559e..9287d70 100644
--- a/test/reactor/executor_test.exs
+++ b/test/reactor/executor_test.exs
@@ -183,9 +183,11 @@ defmodule Reactor.ExecutorTest do
 
       {:ok, agent} = Agent.start_link(fn -> MapSet.new() end)
 
-      assert {:error, ["I fail"]} =
+      assert {:error, error} =
                Reactor.Executor.run(reactor, %{agent: agent}, %{}, max_iterations: 100)
 
+      assert Exception.message(error) =~ "I fail"
+
       effects = Agent.get(agent, & &1)
 
       assert MapSet.size(effects) == 0
diff --git a/test/reactor/planner_test.exs b/test/reactor/planner_test.exs
index 78b017a..3643426 100644
--- a/test/reactor/planner_test.exs
+++ b/test/reactor/planner_test.exs
@@ -1,7 +1,7 @@
 defmodule Reactor.PlannerTest do
   @moduledoc false
   use ExUnit.Case, async: true
-  alias Reactor.{Builder, Error.PlanError, Info, Planner}
+  alias Reactor.{Builder, Error.Internal.PlanError, Info, Planner}
 
   describe "plan/1" do
     test "when the argument is not a reactor, it returns an error" do
diff --git a/test/reactor/step/around_test.exs b/test/reactor/step/around_test.exs
index a34cc81..ef98865 100644
--- a/test/reactor/step/around_test.exs
+++ b/test/reactor/step/around_test.exs
@@ -1,7 +1,7 @@
 defmodule Reactor.Step.AroundTest do
   @moduledoc false
   use ExUnit.Case, async: true
-  alias Reactor.{Builder, Step.Around, Step.ReturnAllArguments}
+  alias Reactor.{Builder, Error.Invalid.MissingInputError, Step.Around, Step.ReturnAllArguments}
 
   setup do
     context = %{current_step: %{name: :marty}}
@@ -27,7 +27,7 @@ defmodule Reactor.Step.AroundTest do
       assert {:error, error} =
                Around.run(%{}, context, Keyword.put(options, :fun, {Marty, :marty, []}))
 
-      assert error =~ ~r/`Marty.marty\/4` to be exported/i
+      assert Exception.message(error) =~ ~r/`Marty.marty\/4` to be exported/i
     end
 
     test "when passed steps which are not steps, it returns an error", %{
@@ -42,8 +42,12 @@ defmodule Reactor.Step.AroundTest do
       context: context,
       options: options
     } do
-      assert {:error, [error]} = Around.run(%{}, context, options)
-      assert error =~ ~r/missing input `arg`/i
+      assert {:error,
+              %{
+                errors: [
+                  %MissingInputError{step: %{name: :example}, argument: %{source: %{name: :arg}}}
+                ]
+              }} = Around.run(%{}, context, options)
     end
 
     test "when the around function fails before calling the callback, it returns an error", %{
diff --git a/test/reactor/step/group_test.exs b/test/reactor/step/group_test.exs
index 07b1820..ab8796b 100644
--- a/test/reactor/step/group_test.exs
+++ b/test/reactor/step/group_test.exs
@@ -1,7 +1,7 @@
 defmodule Reactor.Step.GroupTest do
   @moduledoc false
   use ExUnit.Case, async: true
-  alias Reactor.{Builder, Step.Group, Step.ReturnAllArguments}
+  alias Reactor.{Builder, Error.Invalid.MissingInputError, Step.Group, Step.ReturnAllArguments}
 
   setup do
     context = %{current_step: %{name: :marty}}
@@ -63,8 +63,8 @@ defmodule Reactor.Step.GroupTest do
       context: context,
       options: options
     } do
-      assert {:error, [error]} = Group.run(%{}, context, options)
-      assert error =~ ~r/missing input `arg`/i
+      assert {:error, %{errors: [%MissingInputError{argument: %{name: :arg}}]}} =
+               Group.run(%{}, context, options)
     end
 
     test "when the before function fails, it returns an error", %{
diff --git a/test/reactor/step/transform_all_test.exs b/test/reactor/step/transform_all_test.exs
index c2b48b9..39d9d62 100644
--- a/test/reactor/step/transform_all_test.exs
+++ b/test/reactor/step/transform_all_test.exs
@@ -20,7 +20,7 @@ defmodule Reactor.Step.TransformAllTest do
 
     test "when the function raises, it returns an error" do
       assert {:error, error} = Step.TransformAll.run(%{a: 1}, %{}, fun: fn _ -> raise "hell" end)
-      assert Exception.message(error) == "hell"
+      assert Exception.message(error) =~ "hell"
     end
   end
 end
diff --git a/test/reactor/step/transform_test.exs b/test/reactor/step/transform_test.exs
index e2998bc..e33263f 100644
--- a/test/reactor/step/transform_test.exs
+++ b/test/reactor/step/transform_test.exs
@@ -9,7 +9,7 @@ defmodule Reactor.Step.TransformTest do
 
   describe "run/3" do
     test "when the value argument is missing" do
-      assert {:error, error} = run(%{}, %{}, [])
+      assert {:error, error} = run(%{}, %{current_step: :current_step}, [])
       assert Exception.message(error) =~ "argument is missing"
     end