diff --git a/lib/brex_elixirpb.pb.ex b/lib/brex_elixirpb.pb.ex
new file mode 100644
index 00000000..7124410e
--- /dev/null
+++ b/lib/brex_elixirpb.pb.ex
@@ -0,0 +1,20 @@
+defmodule Brex.Elixirpb.FieldOptions do
+  @moduledoc false
+  use Protobuf, syntax: :proto2
+
+  @type t :: %__MODULE__{
+          extype: String.t()
+        }
+  defstruct [:extype]
+
+  field :extype, 1, optional: true, type: :string
+end
+
+defmodule Brex.Elixirpb.PbExtension do
+  @moduledoc false
+  use Protobuf, syntax: :proto2
+
+  extend Google.Protobuf.FieldOptions, :field, 65007,
+    optional: true,
+    type: Brex.Elixirpb.FieldOptions
+end
diff --git a/lib/protobuf/extension.ex b/lib/protobuf/extension.ex
index 3cc8b34e..afd9d8dd 100644
--- a/lib/protobuf/extension.ex
+++ b/lib/protobuf/extension.ex
@@ -135,6 +135,8 @@ defmodule Protobuf.Extension do
           raise "Extension #{inspect(ext.extendee)}##{fnum} already exists"
         end
 
+        # IO.inspect(:stderr, {fnum_key, mod}, label: :extensions)
+
         GlobalStore.put(fnum_key, mod)
       end)
     end)
diff --git a/lib/protobuf/extension/props.ex b/lib/protobuf/extension/props.ex
index efef629b..6b9947ac 100644
--- a/lib/protobuf/extension/props.ex
+++ b/lib/protobuf/extension/props.ex
@@ -5,7 +5,7 @@ defmodule Protobuf.Extension.Props do
     @moduledoc false
     @type t :: %__MODULE__{
             extendee: module,
-            field_props: FieldProps.T
+            field_props: FieldProps.t()
           }
     defstruct extendee: nil,
               field_props: nil
diff --git a/lib/protobuf/field_props.ex b/lib/protobuf/field_props.ex
index 693bed75..88581f22 100644
--- a/lib/protobuf/field_props.ex
+++ b/lib/protobuf/field_props.ex
@@ -17,7 +17,8 @@ defmodule Protobuf.FieldProps do
           packed?: boolean,
           map?: boolean,
           deprecated?: boolean,
-          encoded_fnum: iodata
+          encoded_fnum: iodata,
+          options: List.t() | nil
         }
   defstruct fnum: nil,
             name: nil,
@@ -34,5 +35,6 @@ defmodule Protobuf.FieldProps do
             packed?: nil,
             map?: false,
             deprecated?: false,
-            encoded_fnum: nil
+            encoded_fnum: nil,
+            options: nil
 end
diff --git a/lib/protobuf/message_props.ex b/lib/protobuf/message_props.ex
index 5afbbe9d..9fe6166a 100644
--- a/lib/protobuf/message_props.ex
+++ b/lib/protobuf/message_props.ex
@@ -6,7 +6,7 @@ defmodule Protobuf.MessageProps do
   @type t :: %__MODULE__{
           ordered_tags: [integer],
           tags_map: %{integer => integer},
-          field_props: %{integer => FieldProps.T},
+          field_props: %{integer => FieldProps.t()},
           field_tags: %{atom => integer},
           repeated_fields: [atom],
           embedded_fields: [atom],
diff --git a/lib/protobuf/protoc/cli.ex b/lib/protobuf/protoc/cli.ex
index 58a15047..f4a9d3aa 100644
--- a/lib/protobuf/protoc/cli.ex
+++ b/lib/protobuf/protoc/cli.ex
@@ -66,6 +66,11 @@ defmodule Protobuf.Protoc.CLI do
     parse_params(ctx, t)
   end
 
+  def parse_params(ctx, ["using_module=" <> using_module | t]) do
+    ctx = %{ctx | using_module: using_module}
+    parse_params(ctx, t)
+  end
+
   def parse_params(ctx, _), do: ctx
 
   @doc false
diff --git a/lib/protobuf/protoc/context.ex b/lib/protobuf/protoc/context.ex
index 9c7dacbf..ce0e4fb2 100644
--- a/lib/protobuf/protoc/context.ex
+++ b/lib/protobuf/protoc/context.ex
@@ -29,7 +29,10 @@ defmodule Protobuf.Protoc.Context do
             gen_descriptors?: false,
 
             # Elixirpb.FileOptions
-            custom_file_options: %{}
+            custom_file_options: %{},
+
+            # Allow custom code injection
+            using_module: "Protobuf"
 
   def cal_file_options(ctx, nil) do
     %{ctx | custom_file_options: %{}, module_prefix: ctx.package || ""}
diff --git a/lib/protobuf/protoc/generator/enum.ex b/lib/protobuf/protoc/generator/enum.ex
index 9c3dd4b6..d258d5f5 100644
--- a/lib/protobuf/protoc/generator/enum.ex
+++ b/lib/protobuf/protoc/generator/enum.ex
@@ -13,7 +13,14 @@ defmodule Protobuf.Protoc.Generator.Enum do
     generate_desc = if ctx.gen_descriptors?, do: desc, else: nil
     type = generate_type(desc.value)
 
-    Protobuf.Protoc.Template.enum(msg_name, msg_opts(ctx, desc), fields, type, generate_desc)
+    Protobuf.Protoc.Template.enum(
+      msg_name,
+      msg_opts(ctx, desc),
+      fields,
+      type,
+      generate_desc,
+      ctx.using_module
+    )
   end
 
   def generate_type(fields) do
diff --git a/lib/protobuf/protoc/generator/extension.ex b/lib/protobuf/protoc/generator/extension.ex
index 8ce1d2b7..e5f19d99 100644
--- a/lib/protobuf/protoc/generator/extension.ex
+++ b/lib/protobuf/protoc/generator/extension.ex
@@ -24,7 +24,7 @@ defmodule Protobuf.Protoc.Generator.Extension do
     else
       name = Util.trans_name(@ext_postfix)
       msg_name = Util.mod_name(ctx, ns ++ [name])
-      Protobuf.Protoc.Template.extension(msg_name, msg_opts(ctx, desc), extends)
+      Protobuf.Protoc.Template.extension(msg_name, msg_opts(ctx, desc), extends, ctx.using_module)
     end
   end
 
diff --git a/lib/protobuf/protoc/generator/message.ex b/lib/protobuf/protoc/generator/message.ex
index 33d631a5..1365c28d 100644
--- a/lib/protobuf/protoc/generator/message.ex
+++ b/lib/protobuf/protoc/generator/message.ex
@@ -34,7 +34,8 @@ defmodule Protobuf.Protoc.Generator.Message do
       fields: fields,
       oneofs: oneofs_str(desc.oneof_decl),
       desc: generate_desc,
-      extensions: extensions
+      extensions: extensions,
+      using_module: ctx.using_module
     }
   end
 
@@ -47,7 +48,8 @@ defmodule Protobuf.Protoc.Generator.Message do
       msg_struct[:oneofs],
       gen_fields(syntax, msg_struct[:fields]),
       msg_struct[:desc],
-      gen_extensions(msg_struct[:extensions])
+      gen_extensions(msg_struct[:extensions]),
+      msg_struct[:using_module]
     )
   end
 
@@ -179,6 +181,7 @@ defmodule Protobuf.Protoc.Generator.Message do
   def get_field(ctx, f, nested_maps, oneofs) do
     opts = field_options(f)
     map = nested_maps[f.type_name]
+
     opts = if map, do: Map.put(opts, :map, true), else: opts
 
     opts =
@@ -288,8 +291,22 @@ defmodule Protobuf.Protoc.Generator.Message do
   end
 
   defp merge_field_options(opts, f) do
+    custom_options =
+      f.options
+      |> Google.Protobuf.FieldOptions.get_extension(Brex.Elixirpb.PbExtension, :field)
+      |> case do
+        nil ->
+          nil
+
+        elixir_field_options ->
+          elixir_field_options
+          |> Map.from_struct()
+          |> Enum.into([])
+      end
+
     opts
     |> Map.put(:packed, f.options.packed)
     |> Map.put(:deprecated, f.options.deprecated)
+    |> Map.put(:options, custom_options)
   end
 end
diff --git a/lib/protobuf/protoc/generator/util.ex b/lib/protobuf/protoc/generator/util.ex
index a9ac87a5..84ddb41e 100644
--- a/lib/protobuf/protoc/generator/util.ex
+++ b/lib/protobuf/protoc/generator/util.ex
@@ -49,5 +49,6 @@ defmodule Protobuf.Protoc.Generator.Util do
   end
 
   def print(v) when is_atom(v), do: inspect(v)
+  def print(v) when is_list(v), do: inspect(v)
   def print(v), do: v
 end
diff --git a/lib/protobuf/protoc/template.ex b/lib/protobuf/protoc/template.ex
index 2a07c454..bdbcba9f 100644
--- a/lib/protobuf/protoc/template.ex
+++ b/lib/protobuf/protoc/template.ex
@@ -11,11 +11,25 @@ defmodule Protobuf.Protoc.Template do
     :def,
     :message,
     @msg_tmpl,
-    [:name, :options, :struct_fields, :typespec, :oneofs, :fields, :desc, :extensions],
+    [
+      :name,
+      :options,
+      :struct_fields,
+      :typespec,
+      :oneofs,
+      :fields,
+      :desc,
+      :extensions,
+      :using_module
+    ],
     trim: true
   )
 
-  EEx.function_from_file(:def, :enum, @enum_tmpl, [:name, :options, :fields, :type, :desc],
+  EEx.function_from_file(
+    :def,
+    :enum,
+    @enum_tmpl,
+    [:name, :options, :fields, :type, :desc, :using_module],
     trim: true
   )
 
@@ -23,5 +37,7 @@ defmodule Protobuf.Protoc.Template do
     trim: true
   )
 
-  EEx.function_from_file(:def, :extension, @ext_tmpl, [:name, :options, :extends], trim: true)
+  EEx.function_from_file(:def, :extension, @ext_tmpl, [:name, :options, :extends, :using_module],
+    trim: true
+  )
 end
diff --git a/priv/templates/enum.ex.eex b/priv/templates/enum.ex.eex
index 690695ad..d38080f0 100644
--- a/priv/templates/enum.ex.eex
+++ b/priv/templates/enum.ex.eex
@@ -1,7 +1,7 @@
 defmodule <%= name %> do
   @moduledoc false
-  use Protobuf<%= options %>
-  
+  use <%= using_module %><%= options %>
+
   <%= type %>
 
   <%= if not is_nil(desc) do %>
diff --git a/priv/templates/extension.ex.eex b/priv/templates/extension.ex.eex
index 3c29c173..da77f9bf 100644
--- a/priv/templates/extension.ex.eex
+++ b/priv/templates/extension.ex.eex
@@ -1,6 +1,6 @@
 defmodule <%= name %> do
   @moduledoc false
-  use Protobuf<%= options %>
+  use <%= using_module %><%= options %>
 
 <%= for ext <- extends do %>  extend <%= ext %>
 <% end %>
diff --git a/priv/templates/message.ex.eex b/priv/templates/message.ex.eex
index 9f0aa76f..9e5c970d 100644
--- a/priv/templates/message.ex.eex
+++ b/priv/templates/message.ex.eex
@@ -1,6 +1,6 @@
 defmodule <%= name %> do
   @moduledoc false
-  use Protobuf<%= options %>
+  use <%= using_module %><%= options %>
 
   <%= typespec %>
   defstruct [<%= struct_fields %>]
diff --git a/src/brex_elixirpb.proto b/src/brex_elixirpb.proto
new file mode 100644
index 00000000..233c5f6a
--- /dev/null
+++ b/src/brex_elixirpb.proto
@@ -0,0 +1,22 @@
+syntax = "proto2";
+
+package brex.elixirpb;
+import "google/protobuf/descriptor.proto";
+
+// Defines an extension to specify the elixir type generated for the given field.
+
+// For example,
+// google.protobuf.StringValue my_string = 1 [(brex.elixirpb.field).extype="String.t"];
+
+// To compile
+//protoc --plugin=/Users/lizard/Projects/Work/git/brex/protobuf-elixir/protoc-gen-elixir --proto_path=lib --elixir_out=lib brex_elixirpb.proto
+
+message FieldOptions {
+  // Specify an elixir type to generate for this field. This will override usual type.
+  optional string extype = 1;
+}
+
+extend google.protobuf.FieldOptions {
+  // Note: number to change
+  optional FieldOptions field = 65007;
+}
\ No newline at end of file
diff --git a/test/protobuf/dsl_test.exs b/test/protobuf/dsl_test.exs
index 12b7e254..8b0e7fbb 100644
--- a/test/protobuf/dsl_test.exs
+++ b/test/protobuf/dsl_test.exs
@@ -223,4 +223,9 @@ defmodule Protobuf.DSLTest do
     assert msg_props.field_props[4].oneof == 1
     refute msg_props.field_props[5].oneof
   end
+
+  test "Extension use case" do
+    msg_props = TestMsg.Ext.UseCase.__message_props__()
+    assert %Protobuf.FieldProps{options: [extype: "String.t"]} = msg_props.field_props[1]
+  end
 end
diff --git a/test/protobuf/protoc/generator/message_test.exs b/test/protobuf/protoc/generator/message_test.exs
index 605b1780..8bb2b9f4 100644
--- a/test/protobuf/protoc/generator/message_test.exs
+++ b/test/protobuf/protoc/generator/message_test.exs
@@ -173,7 +173,42 @@ defmodule Protobuf.Protoc.Generator.MessageTest do
     assert msg =~ "field :a, 1, optional: true, type: :int32, deprecated: true\n"
   end
 
-  test "generete/2 supports message type field" do
+  test "generate/2 supports extensions on field options" do
+    ctx = %Context{package: ""}
+
+    field_opts = Google.Protobuf.FieldOptions.new()
+    custom_opts = Brex.Elixirpb.FieldOptions.new(extype: "String.t")
+
+    opts =
+      Google.Protobuf.FieldOptions.put_extension(
+        field_opts,
+        Brex.Elixirpb.PbExtension,
+        :field,
+        custom_opts
+      )
+
+    desc =
+      Google.Protobuf.DescriptorProto.new(
+        name: "Foo",
+        field: [
+          Google.Protobuf.FieldDescriptorProto.new(
+            name: "my_string",
+            number: 1,
+            type: :TYPE_MESSAGE,
+            # type_name: ".google.protodescbuf.StringValue",
+            label: :LABEL_OPTIONAL,
+            options: opts
+          )
+        ]
+      )
+
+    {[], [msg]} = Generator.generate(ctx, desc)
+
+    assert msg =~
+             "field :my_string, 1, optional: true, type: :message, options: [extype: \"String.t\"]\n"
+  end
+
+  test "generate/2 supports message type field" do
     ctx = %Context{
       package: "",
       dep_type_mapping: %{
diff --git a/test/support/test_msg.ex b/test/support/test_msg.ex
index 9d60154d..27d8f515 100644
--- a/test/support/test_msg.ex
+++ b/test/support/test_msg.ex
@@ -212,4 +212,16 @@ defmodule TestMsg do
     extend Ext.Foo2, :bar, 1047, optional: true, type: :string
     extend Ext.Foo1, :"Parent.foo", 1048, optional: true, type: Ext.EnumFoo, enum: true
   end
+
+  defmodule Ext.UseCase do
+    @moduledoc false
+    use Protobuf, syntax: :proto3
+
+    @type t :: %__MODULE__{
+            my_string: Google.Protobuf.StringValue.t() | nil
+          }
+    defstruct [:my_string]
+
+    field :my_string, 1, type: Google.Protobuf.StringValue, options: [extype: "String.t"]
+  end
 end