diff --git a/.yardopts b/.yardopts
new file mode 100644
index 0000000..a4541be
--- /dev/null
+++ b/.yardopts
@@ -0,0 +1 @@
+-m markdown
diff --git a/README.markdown b/README.markdown
index d53b568..06993bd 100644
--- a/README.markdown
+++ b/README.markdown
@@ -65,6 +65,9 @@ counterparts. You can also include just stringify or just symbolize with
 
 ### MergeInitializer
 
+The MergeInitializer extension simply makes it possible to initialize a
+Hash subclass with another Hash, giving you a quick short-hand.
+
 ### MethodAccess
 
 The MethodAccess extension allows you to quickly build method-based
@@ -81,17 +84,24 @@ included as individual modules, i.e. `Hashie::Extensions::MethodReader`,
     h.abc  # => 'def'
     h.abc? # => true
 
+### IndifferentAccess
+
+This extension can be mixed in to instantly give you indifferent access
+to your Hash subclass. This works just like the params hash in Rails and
+other frameworks where whether you provide symbols or strings to access
+keys, you will get the same results.
+
+A unique feature of Hashie's IndifferentAccess mixin is that it will
+inject itself recursively into subhashes *without* reinitializing the
+hash in question. This means you can safely merge together indifferent
+and non-indifferent hashes arbitrarily deeply without worrying about
+whether you'll be able to `hash[:other][:another]` properly.
+
 ### DeepMerge (Unimplemented)
 
 This extension *will* allow you to easily include a recursive merging
 system to any Hash descendant.
 
-### IndifferentAccess (Unimplemented)
-
-This extension *will* allow you to easily give a hash rules for
-normalizing keys, for instance to allow symbol or string keys both to
-reach the intended value.
-
 ## Mash
 
 Mash is an extended Hash that gives simple pseudo-object functionality
diff --git a/lib/hashie/extensions/indifferent_access.rb b/lib/hashie/extensions/indifferent_access.rb
index 6635a1d..50d4081 100644
--- a/lib/hashie/extensions/indifferent_access.rb
+++ b/lib/hashie/extensions/indifferent_access.rb
@@ -1,7 +1,110 @@
 module Hashie
   module Extensions
+    # IndifferentAccess gives you the ability to not care
+    # whether your hash has string or symbol keys. Made famous
+    # in Rails for accessing query and POST parameters, this
+    # is a handy tool for making sure your hash has maximum
+    # utility.
+    #
+    # One unique feature of this mixin is that it will recursively
+    # inject itself into sub-hash instances without modifying
+    # the actual class of the sub-hash.
+    #
+    # @example
+    #   class MyHash < Hash
+    #     include Hashie::Extensions::MergeInitializer
+    #     include Hashie::Extensions::IndifferentAccess
+    #   end
+    #
+    #   h = MyHash.new(:foo => 'bar', 'baz' => 'blip')
+    #   h['foo'] # => 'bar'
+    #   h[:foo]  # => 'bar'
+    #   h[:baz]  # => 'blip'
+    #   h['baz'] # => 'blip'
+    #
     module IndifferentAccess
-      # TODO: Implement indifferent access.
+      def self.included(base)
+        base.class_eval do
+          alias_method :regular_writer, :[]=
+          alias_method :[]=, :indifferent_writer
+          %w(default update fetch delete key? values_at).each do |m|
+            alias_method "regular_#{m}", m
+            alias_method m, "indifferent_#{m}"
+          end
+        end
+      end
+
+      # This will inject indifferent access into an instance of
+      # a hash without modifying the actual class. This is what
+      # allows IndifferentAccess to spread to sub-hashes.
+      def self.inject!(hash)
+        hash.singleton_class.send :include, Hashie::Extensions::IndifferentAccess
+        hash.convert!
+      end
+
+      # Injects indifferent access into a duplicate of the hash
+      # provided. See #inject!
+      def self.inject(hash)
+        inject!(hash.dup)
+      end
+
+      def convert_key(key)
+        key.to_s
+      end
+
+      # Iterates through the keys and values, reconverting them to
+      # their proper indifferent state. Used when IndifferentAccess
+      # is injecting itself into member hashes.
+      def convert!
+        keys.each do |k|
+          regular_writer convert_key(k), convert_value(self.regular_delete(k))
+        end
+        self
+      end
+
+      def convert_value(value)
+        if hash_lacking_indifference?(value)
+          Hashie::Extensions::IndifferentAccess.inject(value.dup)
+        elsif value.is_a?(::Array)
+          value.dup.replace(value.map { |e| convert_value(e) })
+        else
+          value
+        end
+      end
+      
+      def indifferent_default(key = nil)
+        return self[convert_key(key)] if key?(key)
+        regular_default(key)
+      end
+
+      def indifferent_update(other_hash)
+        return regular_update(other_hash) if hash_with_indifference?(other_hash)
+        other_hash.each_pair do |k,v|
+          self[k] = v
+        end
+      end
+      
+      def indifferent_writer(key, value);  regular_writer convert_key(key), convert_value(value) end
+      def indifferent_fetch(key, *args);   regular_fetch  convert_key(key), *args                end
+      def indifferent_delete(key);         regular_delete convert_key(key)                       end
+      def indifferent_key?(key);           regular_key?   convert_key(key)                       end
+      def indifferent_values_at(*indices); indices.map{|i| self[i] }                             end
+
+      def indifferent_access?; true end
+      
+      protected
+
+      def hash_lacking_indifference?(other)
+        other.is_a?(::Hash) &&
+        !(other.respond_to?(:indifferent_access?) &&
+          other.indifferent_access?)
+      end
+
+      def hash_with_indifference?(other)
+        other.is_a?(::Hash) &&
+        other.respond_to?(:indifferent_access?) &&
+        other.indifference_access?
+      end
     end
   end
 end
diff --git a/spec/hashie/extensions/indifferent_access_spec.rb b/spec/hashie/extensions/indifferent_access_spec.rb
index 12ba044..7c421cb 100644
--- a/spec/hashie/extensions/indifferent_access_spec.rb
+++ b/spec/hashie/extensions/indifferent_access_spec.rb
@@ -1,5 +1,66 @@
 require 'spec_helper'
 
 describe Hashie::Extensions::IndifferentAccess do
-  
+  class IndifferentHash < Hash
+    include Hashie::Extensions::MergeInitializer
+    include Hashie::Extensions::IndifferentAccess
+  end
+  subject{ IndifferentHash }
+
+  it 'should be able to access via string or symbol' do
+    h = subject.new(:abc => 123)
+    h[:abc].should == 123
+    h['abc'].should == 123
+  end
+
+  describe '#values_at' do
+    it 'should indifferently find values' do
+      h = subject.new(:foo => 'bar', 'baz' => 'qux')
+      h.values_at('foo', :baz).should == %w(bar qux)
+    end
+  end
+
+  describe '#fetch' do
+    it 'should work like normal fetch, but indifferent' do
+      h = subject.new(:foo => 'bar')
+      h.fetch(:foo).should == h.fetch('foo')
+      h.fetch(:foo).should == 'bar'
+    end
+  end
+
+  describe '#delete' do
+    it 'should delete indifferently' do
+      h = subject.new(:foo => 'bar', 'baz' => 'qux')
+      h.delete('foo')
+      h.delete(:baz)
+      h.should be_empty
+    end
+  end
+
+  describe '#key?' do
+    it 'should find it indifferently' do
+      h = subject.new(:foo => 'bar')
+      h.should be_key(:foo)
+      h.should be_key('foo')
+    end
+  end
+
+  describe '#update' do
+    subject{ IndifferentHash.new(:foo => 'bar') }
+    it 'should allow keys to be indifferent still' do
+      subject.update(:baz => 'qux')
+      subject['foo'].should == 'bar'
+      subject['baz'].should == 'qux'
+    end
+
+    it 'should recursively inject indifference into sub-hashes' do
+      subject.update(:baz => {:qux => 'abc'})
+      subject['baz']['qux'].should == 'abc'
+    end
+
+    it 'should not change the ancestors of the injected object class' do
+      subject.update(:baz => {:qux => 'abc'})
+      Hash.new.should_not be_respond_to(:indifferent_access?)
+    end
+  end
 end