diff --git a/lib/plug.ex b/lib/plug.ex
index 4fd2da7a..a2d25813 100644
--- a/lib/plug.ex
+++ b/lib/plug.ex
@@ -40,6 +40,8 @@ defmodule Plug do
   Here's an example of a module plug:
 
       defmodule JSONHeaderPlug do
+        @behaviour Plug
+
         import Plug.Conn
 
         def init(opts) do
@@ -143,6 +145,8 @@ defmodule Plug do
   ## Example
 
       defmodule Router do
+        @behaviour Plug
+
         def init(opts), do: opts
 
         def call(conn, opts) do