diff --git a/lib/aws_codegen.ex b/lib/aws_codegen.ex
index a614383..5be5a84 100644
--- a/lib/aws_codegen.ex
+++ b/lib/aws_codegen.ex
@@ -33,6 +33,7 @@ defmodule AWS.CodeGen do
               protocol: nil,
               signature_version: nil,
               service_id: nil,
+              shapes: %{},
               signing_name: nil,
               target_prefix: nil
   end
diff --git a/lib/aws_codegen/post_service.ex b/lib/aws_codegen/post_service.ex
index c32e4bd..9a897c9 100644
--- a/lib/aws_codegen/post_service.ex
+++ b/lib/aws_codegen/post_service.ex
@@ -1,15 +1,31 @@
 defmodule AWS.CodeGen.PostService do
   alias AWS.CodeGen.Docstring
   alias AWS.CodeGen.Service
+  alias AWS.CodeGen.Shapes
+  alias AWS.CodeGen.Name
 
   defmodule Action do
     defstruct arity: nil,
               docstring: nil,
               function_name: nil,
+              input: nil,
+              output: nil,
+              errors: %{},
               host_prefix: nil,
               name: nil
   end
 
+  defmodule Shape do
+    defstruct name: nil,
+              type: nil,
+              members: [],
+              member: [],
+              enum: [],
+              min: nil,
+              required: [],
+              is_input: nil
+  end
+
   @configuration %{
     "ec2" => %{
       content_type: "application/x-www-form-urlencoded",
@@ -57,6 +73,7 @@ defmodule AWS.CodeGen.PostService do
     service = spec.api["shapes"][spec.shape_name]
     traits = service["traits"]
     actions = collect_actions(language, spec.api)
+    shapes = collect_shapes(language, spec.api)
     endpoint_prefix = traits["aws.api#service"]["endpointPrefix"] || traits["aws.api#service"]["arnNamespace"]
     endpoint_info = endpoints_spec["services"][endpoint_prefix]
     is_global = not is_nil(endpoint_info) and not Map.get(endpoint_info, "isRegionalized", true)
@@ -89,6 +106,7 @@ defmodule AWS.CodeGen.PostService do
       language: language,
       module_name: spec.module_name,
       protocol: protocol |> to_string() |> String.replace("_", "-"),
+      shapes: shapes,
       signing_name: signing_name,
       signature_version: AWS.CodeGen.Util.get_signature_version(service),
       service_id: AWS.CodeGen.Util.get_service_id(service),
@@ -137,10 +155,45 @@ defmodule AWS.CodeGen.PostService do
           ),
         function_name: AWS.CodeGen.Name.to_snake_case(operation),
         host_prefix: operation_spec["traits"]["smithy.api#endpoint"]["hostPrefix"],
-        name: String.replace(operation, ~r/com\.amazonaws\.[^#]+#/, "")
+        name: String.replace(operation, ~r/com\.amazonaws\.[^#]+#/, ""),
+        input: operation_spec["input"],
+        output: operation_spec["output"],
+        errors: operation_spec["errors"]
       }
     end)
     |> Enum.sort(fn a, b -> a.function_name < b.function_name end)
     |> Enum.uniq()
   end
+
+  defp collect_shapes(_language, api_spec) do
+    api_spec["shapes"]
+    |> Enum.sort(fn {name_a, _}, {name_b, _} -> name_a < name_b end)
+    |> Enum.map(fn {name, shape} ->
+      {name,
+       %Shape{
+         name: name,
+         type: shape["type"],
+         member: shape["member"],
+         members: shape["members"],
+         min: shape["min"],
+         enum: shape["enum"],
+         is_input: is_input?(shape)
+       }}
+    end)
+    |> Enum.into(%{})
+  end
+
+  defp is_input?(shape) do
+    if Map.has_key?(shape, "traits") do
+      traits = shape["traits"]
+      if Map.has_key?(traits, "smithy.api#input") do
+        true
+      else
+        false
+      end
+    else
+      true
+    end
+  end
+
 end
diff --git a/lib/aws_codegen/shapes.ex b/lib/aws_codegen/shapes.ex
index 946f9cc..ab395d6 100644
--- a/lib/aws_codegen/shapes.ex
+++ b/lib/aws_codegen/shapes.ex
@@ -1,4 +1,5 @@
 defmodule AWS.CodeGen.Shapes do
+  alias AWS.CodeGen.Name
   @moduledoc false
 
   def get_input_shape(operation_spec) do
diff --git a/lib/aws_codegen/types.ex b/lib/aws_codegen/types.ex
new file mode 100644
index 0000000..7a6c5dd
--- /dev/null
+++ b/lib/aws_codegen/types.ex
@@ -0,0 +1,150 @@
+defmodule AWS.CodeGen.Types do
+  alias AWS.CodeGen.PostService.Shape
+  alias AWS.CodeGen.Name
+
+  # Unfortunately, gotta patch over auto-defining types that already exist in Elixir
+
+  def shape_to_type(:elixir, "String", _) do
+    "String.t()"
+  end
+  def shape_to_type(:erlang, "String", _) do
+    "string()"
+  end
+
+  def shape_to_type(:elixir, "string", _) do
+    "String.t()"
+  end
+  def shape_to_type(:erlang, "string", _) do
+    "string()"
+  end
+
+  def shape_to_type(:elixir, "Identifier", _) do
+    "String.t()"
+  end
+  def shape_to_type(:erlang, "Identifier", _) do
+    "string()"
+  end
+
+  def shape_to_type(:elixir, "identifier", _) do
+    "String.t()"
+  end
+  def shape_to_type(:erlang, "identifier", _) do
+    "string()"
+  end
+
+  def shape_to_type(:elixir, "XmlString" <> _rest, _) do
+    "String.t()"
+  end
+  def shape_to_type(:erlang, "XmlString" <> _rest, _) do
+    "string"
+  end
+
+  def shape_to_type(:elixir, "NullablePositiveInteger", _) do
+    "nil | non_neg_integer()"
+  end
+  def shape_to_type(:erlang, "NullablePositiveInteger", _) do
+    "undefined | non_neg_integer()"
+  end
+
+  def shape_to_type(_, %Shape{type: type}, _module_name) when type in ["float", "double", "long"] do
+    "float()"
+  end
+
+  def shape_to_type(_, %Shape{type: "timestamp"}, _module_name) do
+    "non_neg_integer()"
+  end
+
+  def shape_to_type(_, %Shape{type: "map"}, _module_name) do
+    "map()"
+  end
+
+  def shape_to_type(_, %Shape{type: "blob"}, _module_name) do
+    "binary()"
+  end
+
+  def shape_to_type(:elixir, %Shape{type: "string"}, _module_name) do
+    "String.t()"
+  end
+  def shape_to_type(:erlang, %Shape{type: "string"}, _module_name) do
+    "string()"
+  end
+
+  def shape_to_type(_, %Shape{type: "integer"}, _module_name) do
+    "integer()"
+  end
+
+  def shape_to_type(_, %Shape{type: "boolean"}, _module_name) do
+    "boolean()"
+  end
+
+  def shape_to_type(_, %Shape{type: "enum"}, _module_name) do
+    "list(any())"
+  end
+
+  def shape_to_type(_, %Shape{type: "union"}, _module_name) do
+    "list()"
+  end
+
+  def shape_to_type(_, %Shape{type: "document"}, _module_name) do
+    "any()"
+  end
+
+  def shape_to_type(context, shape_name, module_name, all_shapes) do
+    case shape_name do
+      "smithy.api#String" ->
+        "[#{shape_to_type(context.language, %Shape{type: "string"}, module_name)}]"
+
+      "smithy.api#Integer" ->
+        "[#{shape_to_type(context.language, %Shape{type: "integer"}, module_name)}]"
+
+      "smithy.api#Timestamp" ->
+        "[#{shape_to_type(context.language, %Shape{type: "timestamp"}, module_name)}]"
+
+      "smithy.api#PrimitiveLong" ->
+        "[#{shape_to_type(context.language, %Shape{type: "long"}, module_name)}]"
+
+      "smithy.api#Long" ->
+        "[#{shape_to_type(context.language, %Shape{type: "long"}, module_name)}]"
+
+      "smithy.api#Boolean" ->
+        "[#{shape_to_type(context.language, %Shape{type: "boolean"}, module_name)}]"
+
+      "smithy.api#PrimitiveBoolean" ->
+        "[#{shape_to_type(context.language, %Shape{type: "boolean"}, module_name)}]"
+
+      "smithy.api#Double" ->
+        "[#{shape_to_type(context.language, %Shape{type: "double"}, module_name)}]"
+
+      "smithy.api#Document" ->
+        "[#{shape_to_type(context.language, %Shape{type: "document"}, module_name)}]"
+
+      "smithy.api#Unit" ->
+        "[]"
+
+      _ ->
+        case all_shapes[shape_name] do
+          %Shape{type: "structure"} ->
+            type = "#{AWS.CodeGen.Name.to_snake_case(String.replace(shape_name, ~r/com\.amazonaws\.[^#]+#/, ""))}"
+            if AWS.CodeGen.Util.reserved_type(type) do
+              "#{String.downcase(String.replace(context.module_name, ["aws_", "AWS."], ""))}_#{type}()"
+            else
+              "#{type}()"
+            end
+
+          %Shape{type: "list", member: member} ->
+            type = "#{shape_to_type(context, member["target"], module_name, all_shapes)}"
+            if AWS.CodeGen.Util.reserved_type(type) do
+              "list(#{String.downcase(String.replace(context.module_name, ["aws_", "AWS."], ""))}_#{type}())"
+            else
+              "list(#{type}())"
+            end
+
+          nil ->
+            raise "Tried to reference an undefined shape for #{shape_name}"
+
+          shape ->
+            shape_to_type(context.language, shape, module_name)
+        end
+    end
+  end
+end
diff --git a/lib/aws_codegen/util.ex b/lib/aws_codegen/util.ex
index 9c416fa..46fb073 100644
--- a/lib/aws_codegen/util.ex
+++ b/lib/aws_codegen/util.ex
@@ -30,4 +30,136 @@ defmodule AWS.CodeGen.Util do
     service["traits"]["aws.api#service"]["sdkId"]
   end
 
+  def input_keys(action, context) do
+    shapes = context.shapes
+    input_shape = action.input["target"]
+    maybe_shape = Enum.filter(shapes, fn {name, _shape} -> input_shape == name end)
+    case maybe_shape do
+      [] ->
+        []
+      [{_name, shape}] ->
+        Enum.reduce(shape.members,
+                    [],
+                    fn {name, %{"traits" => traits}}, acc ->
+                        if Map.has_key?(traits, "smithy.api#required") do
+                          [name <> " Required: true" | acc]
+                        else
+                          [name <> " Required: false" | acc]
+                        end
+                       {name, _shape}, acc ->
+                          [name <> " Required: false" | acc]
+                    end)
+        |> Enum.reverse()
+    end
+  end
+
+  def types(context) do
+    Enum.reduce(context.shapes,
+                Map.new(),
+                fn {_name, shape}, acc ->
+                  if shape.type == "structure" and not is_nil(shape.members) do
+                    type = AWS.CodeGen.Name.to_snake_case(String.replace(shape.name, ~r/com\.amazonaws\.[^#]+#/, ""))
+                    types = Enum.reduce(shape.members,
+                                        Map.new(),
+                                        fn {_name, shape_member}, a ->
+                                          target = shape_member["target"]
+                                          shape_member_type = AWS.CodeGen.Types.shape_to_type(context, target, context.module_name, context.shapes)
+                                          Map.put(a, is_required(context.language, shape.is_input, shape_member, target), shape_member_type)
+                                        end)
+                    if reserved_type(type) do
+                      Map.put(acc, "#{String.downcase(String.replace(context.module_name, "AWS.", ""))}_#{type}", types)
+                    else
+                      Map.put(acc, type, types)
+                    end
+                  else
+                    acc
+                  end
+                end)
+  end
+
+  defp is_required(:elixir, is_input, shape, target) do
+    trimmed_name = String.replace(target, ~r/com\.amazonaws\.[^#]+#/, "")
+    if is_input do
+      if Map.has_key?(shape, "traits") do
+        if Map.has_key?(shape["traits"], "smithy.api#required") do
+          "required(\"#{trimmed_name}\")"
+        else
+          "optional(\"#{trimmed_name}\")"
+        end
+      else
+        "optional(\"#{trimmed_name}\")"
+      end
+    else
+      "\"#{trimmed_name}\""
+    end
+  end
+  defp is_required(:erlang, _is_input, _shape, target) do
+    # There is no optional/1 | required/1 type of thing in Erlang.
+    # Instead we'd use := | => but let's deal with that problem later...
+    trimmed_name = String.replace(target, ~r/com\.amazonaws\.[^#]+#/, "")
+    "\"#{trimmed_name}\""
+  end
+
+  def function_argument_type(:elixir, action) do
+    case Map.get(action.input, "target") do
+      "smithy.api#Unit" -> "%{}"
+      type ->
+        "#{AWS.CodeGen.Name.to_snake_case(String.replace(type, ~r/com\.amazonaws\.[^#]+#/, ""))}()"
+    end
+  end
+  def function_argument_type(:erlang, action) do
+    case Map.get(action.input, "target") do
+      "smithy.api#Unit" -> "\#{}"
+      type ->
+        "#{AWS.CodeGen.Name.to_snake_case(String.replace(type, ~r/com\.amazonaws\.[^#]+#/, ""))}()"
+    end
+  end
+
+  def return_type(action) do
+    case Map.get(action.output, "target") do
+      "smithy.api#Unit" -> "[]"
+      type ->
+        normal = "{:ok, #{AWS.CodeGen.Name.to_snake_case(String.replace(type, ~r/com\.amazonaws\.[^#]+#/, ""))}(), any()}"
+        errors =
+          if is_list(action.errors) do
+            Enum.map(action.errors,
+            fn %{"target" => error_type} ->
+              "{:error, #{AWS.CodeGen.Name.to_snake_case(String.replace(error_type, ~r/com\.amazonaws\.[^#]+#/, ""))}()}"
+              _ ->
+                ""
+            end )
+          else
+            []
+          end
+        Enum.join([normal, "{:error, {:unexpected_response, any()}}" | errors], " | ")
+    end
+  end
+  def return_type(:erlang, action) do
+    case Map.get(action.output, "target") do
+      "smithy.api#Unit" -> "[]"
+      type ->
+        normal = "{ok, #{AWS.CodeGen.Name.to_snake_case(String.replace(type, ~r/com\.amazonaws\.[^#]+#/, ""))}(), tuple()}"
+        errors =
+          if is_list(action.errors) do
+            Enum.map(action.errors,
+            fn %{"target" => error_type} ->
+              "{error, #{AWS.CodeGen.Name.to_snake_case(String.replace(error_type, ~r/com\.amazonaws\.[^#]+#/, ""))}(), tuple()}"
+              _ ->
+                ""
+            end )
+          else
+            []
+          end
+        Enum.join([normal, "{error, any()}" | errors], " |\n    ")
+    end
+  end
+
+  def reserved_type(type) do
+    if type == "node" || type == "term" do
+      true
+    else
+      false
+    end
+  end
+
 end
diff --git a/priv/post.erl.eex b/priv/post.erl.eex
index 691b1f3..f71b3ae 100644
--- a/priv/post.erl.eex
+++ b/priv/post.erl.eex
@@ -8,11 +8,23 @@
 
 -include_lib("hackney/include/hackney_lib.hrl").
 
+<%= for {type_name, type_fields} <- AWS.CodeGen.Util.types(context) do %>
+%% Example:
+%% <%= type_name %>() :: #{
+<%= Enum.map_join(type_fields, ",\n", fn {field_name, field_type} ->
+  ~s{%%   <<#{field_name}>> => #{field_type}}
+end) %>
+%% }
+-type <%= "#{type_name}()" %> :: #{binary() => any()}.
+<% end %>
+
 %%====================================================================
 %% API
 %%====================================================================
 <%= for action <- context.actions do %>
 <%= action.docstring %>
+-spec <%= action.function_name %>(map(), <%= AWS.CodeGen.Util.function_argument_type(context.language, action)%>, list()) ->
+    <%= AWS.CodeGen.Util.return_type(context.language, action)%>.
 <%= action.function_name %>(Client, Input)
   when is_map(Client), is_map(Input) ->
     <%= action.function_name %>(Client, Input, []).
diff --git a/priv/post.ex.eex b/priv/post.ex.eex
index fc8bd43..dfe1de1 100644
--- a/priv/post.ex.eex
+++ b/priv/post.ex.eex
@@ -11,6 +11,19 @@ defmodule <%= context.module_name %> do
   alias AWS.Client
   alias AWS.Request
 
+ <%= for {type_name, type_fields} <- AWS.CodeGen.Util.types(context) do %>
+@typedoc """
+
+## Example:
+<%= type_name %>() :: %{
+<%= Enum.map_join(type_fields, ",\n", fn {field_name, field_type} ->
+  ~s{  #{field_name} => #{field_type}}
+end) %>
+}
+"""
+@type <%= "#{type_name}()" %> :: %{String.t => any()}
+<% end %>
+
   def metadata do
     %{
       api_version: <%= inspect(context.api_version) %>,
@@ -30,6 +43,7 @@ defmodule <%= context.module_name %> do
   @doc """
 <%= action.docstring %>
   """<% end %>
+  @spec <%= action.function_name %>(map(), <%= AWS.CodeGen.Util.function_argument_type(context.language, action)%>, list()) :: <%= AWS.CodeGen.Util.return_type(context.language, action)%>
   def <%= action.function_name %>(%Client{} = client, input, options \\ []) do
     meta =
       <%= if action.host_prefix do %>
diff --git a/priv/rest.erl.eex b/priv/rest.erl.eex
index cfa3a4d..5c6f003 100644
--- a/priv/rest.erl.eex
+++ b/priv/rest.erl.eex
@@ -135,7 +135,7 @@
 %% Internal functions
 %%====================================================================
 
--spec proplists_take(any(), proplists:proplists(), any()) -> {any(), proplists:proplists()}.
+-spec proplists_take(any(), proplists:proplist(), any()) -> {any(), proplists:proplist()}.
 proplists_take(Key, Proplist, Default) ->
   Value = proplists:get_value(Key, Proplist, Default),
   {Value, proplists:delete(Key, Proplist)}.