diff --git a/lib/rack/reverse_proxy.rb b/lib/rack/reverse_proxy.rb
index 4d63623..e731c20 100644
--- a/lib/rack/reverse_proxy.rb
+++ b/lib/rack/reverse_proxy.rb
@@ -27,6 +27,16 @@ def call(env)
       headers['HOST'] = uri.host if all_opts[:preserve_host]       
       headers['X-Forwarded-Host'] = rackreq.host if all_opts[:x_forwarded_host]
       puts "Proxying #{rackreq.url} => #{uri} (Headers: #{headers.inspect})" if all_opts[:debug]
+      
+      if headers_opt = all_opts[:headers]
+        if headers_opt.is_a?(Proc)
+          headers = headers_opt.call(headers)
+        elsif headers_opt.is_a?(Hash)
+          headers.merge!(headers_opt)
+        else
+          $stderr.puts "Warning: :headers option provided with unsupported type '#{headers_opt.class}'. Expected a Hash or Proc"
+        end
+      end
 
       session = Net::HTTP.new(uri.host, uri.port)
       session.read_timeout=all_opts[:timeout] if all_opts[:timeout]
@@ -37,7 +47,8 @@ def call(env)
       else
         # DO NOT DO THIS IN PRODUCTION !!!
         session.verify_mode = OpenSSL::SSL::VERIFY_NONE
-      end
+      end            
+      
       session.start { |http|
         m = rackreq.request_method
         case m
diff --git a/spec/rack/reverse_proxy_spec.rb b/spec/rack/reverse_proxy_spec.rb
index 4296927..0b6db7d 100644
--- a/spec/rack/reverse_proxy_spec.rb
+++ b/spec/rack/reverse_proxy_spec.rb
@@ -120,6 +120,57 @@ def app
         get '/test/stuff'
       end
     end
+    
+    describe "with headers dictionary provided" do
+      context 'when the header does not exist in the source request' do
+        def app
+          Rack::ReverseProxy.new(dummy_app) do
+            reverse_proxy '/test', 'http://example.com/', {:headers => {'X-EXAMPLE-HEADER' => 'A value'}}
+          end
+        end
+        
+        it "should add the provided headers" do
+          stub_request(:any, 'example.com/test/stuff')
+          get '/test/stuff'
+          a_request(:get, 'http://example.com/test/stuff').with(:headers => {"X-EXAMPLE-HEADER" => "A value"}).should have_been_made
+        end
+      end
+      
+      context 'when the header already appears in the source request' do
+        def app
+          Rack::ReverseProxy.new(dummy_app) do
+            reverse_proxy '/test', 'http://example.com/', {:headers => {'X-EXAMPLE-HEADER' => 'New value'}}
+          end
+        end
+        
+        it "should replace the existing Headers value" do
+          headers = {'Accept'=>'*/*', 'Cookie'=>'', 'Host'=>'example.com', 'User-Agent'=>'Ruby', 
+                     'X-Example-Header'=>'New value', 'X-Forwarded-Host'=>'example.org'}
+          stub_request(:any, 'example.com/test/stuff').with(:headers => headers)
+          get '/test/stuff', {}, {"X-EXAMPLE-HEADER" => "Original value"}
+          a_request(:get, 'http://example.com/test/stuff').with(:headers => {"X-EXAMPLE-HEADER" => "New value"}).should have_been_made
+        end
+      end
+    end
+    
+    describe 'with headers proc provided' do
+      def app
+        Rack::ReverseProxy.new(dummy_app) do
+          reverse_proxy('/test', 'http://example.com/', :headers => lambda do |headers|
+            headers['X-FORWARDED-FOR'].gsub!(', 127.0.0.1', '') if headers['X-FORWARDED-FOR']
+            headers
+          end)
+        end
+      end
+      
+      it "should yield the headers to the proc for manipulation" do
+        headers = {'Accept'=>'*/*', 'Cookie'=>'', 'Host'=>'example.com', 'User-Agent'=>'Ruby', 
+                   'X-Forwarded_For'=>'174.254.197.191', 'X-Forwarded-Host'=>'example.org'}
+        stub_request(:any, 'example.com/test/stuff').with(:headers => headers)
+        get '/test/stuff', {}, {'HTTP_X_Forwarded_For'=>'174.254.197.191, 127.0.0.1'}
+        a_request(:get, 'http://example.com/test/stuff').with(:headers => {"X-Forwarded-For" => "174.254.197.191"}).should have_been_made
+      end
+    end
 
     describe "with ambiguous routes and all matching" do
       def app