diff --git a/README.md b/README.md
index 4385559..b4289d3 100644
--- a/README.md
+++ b/README.md
@@ -84,6 +84,7 @@ request.env['tracker'] = {
 
 * `:anonymize_ip` -  sets the tracker to remove the last octet from all IP addresses, see https://developers.google.com/analytics/devguides/collection/gajs/methods/gaJSApi_gat?hl=de#_gat._anonymizeIp for details.
 * `:cookie_domain` -  sets the domain name for the GATC cookies. Defaults to `auto`.
+* `:user_id` -  defines a proc to set the [userId](https://developers.google.com/analytics/devguides/collection/analyticsjs/user-id). Ex: `user_id: lambda { |env| env['rack.session']['user_id'] }` would return the user_id from the session.
 * `:site_speed_sample_rate` - Defines a new sample set size for Site Speed data collection, see https://developers.google.com/analytics/devguides/collection/gajs/methods/gaJSApiBasicConfiguration?hl=de#_gat.GA_Tracker_._setSiteSpeedSampleRate
 * `:adjusted_bounce_rate_timeouts` - An array of times in seconds that the tracker will use to set timeouts for adjusted bounce rate tracking. See http://analytics.blogspot.ca/2012/07/tracking-adjusted-bounce-rate-in-google.html for details.
 * `:enhanced_link_attribution` - Enables [Enhanced Link Attribution](https://developers.google.com/analytics/devguides/collection/analyticsjs/advanced#enhancedlink).
diff --git a/lib/rack/tracker/google_analytics/google_analytics.rb b/lib/rack/tracker/google_analytics/google_analytics.rb
index 9ca7f19..58965e9 100644
--- a/lib/rack/tracker/google_analytics/google_analytics.rb
+++ b/lib/rack/tracker/google_analytics/google_analytics.rb
@@ -1,4 +1,7 @@
 class Rack::Tracker::GoogleAnalytics < Rack::Tracker::Handler
+
+  ALLOWED_TRACKER_OPTIONS = [:cookie_domain, :user_id]
+
   class Send < OpenStruct
     def initialize(attrs = {})
       attrs.reverse_merge!(type: 'event')
@@ -28,6 +31,18 @@ def tracker
     options[:tracker].respond_to?(:call) ? options[:tracker].call(env) : options[:tracker]
   end
 
+  def tracker_options
+    @tracker_options ||= begin
+      tracker_options = {}
+      options.slice(*ALLOWED_TRACKER_OPTIONS).each do |key, value|
+        if option_value = value.respond_to?(:call) ? value.call(env) : value
+          tracker_options["#{key}".camelize(:lower).to_sym] = "#{option_value}"
+        end
+      end
+      tracker_options
+    end
+  end
+
   def render
     Tilt.new( File.join( File.dirname(__FILE__), 'template', 'google_analytics.erb') ).render(self)
   end
diff --git a/lib/rack/tracker/google_analytics/template/google_analytics.erb b/lib/rack/tracker/google_analytics/template/google_analytics.erb
index 4a2ccdb..511e5d5 100644
--- a/lib/rack/tracker/google_analytics/template/google_analytics.erb
+++ b/lib/rack/tracker/google_analytics/template/google_analytics.erb
@@ -7,7 +7,7 @@
     m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
     })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
 
-    ga('create', '<%= tracker %>', <%= (options[:cookie_domain] ? {cookieDomain: options[:cookie_domain]} : {}).to_json %>);
+    ga('create', '<%= tracker %>', <%= tracker_options.to_json %>);
   }
 
 <% if options[:enhanced_link_attribution] %>
diff --git a/spec/handler/google_analytics_spec.rb b/spec/handler/google_analytics_spec.rb
index 67a67b8..6960d4a 100644
--- a/spec/handler/google_analytics_spec.rb
+++ b/spec/handler/google_analytics_spec.rb
@@ -1,7 +1,10 @@
 RSpec.describe Rack::Tracker::GoogleAnalytics do
 
   def env
-    {misc: 'foobar'}
+    {
+      misc: 'foobar',
+      user_id: '123'
+    }
   end
 
   it 'will be placed in the head' do
@@ -30,6 +33,44 @@ def env
     end
   end
 
+  describe '#tracker_options' do
+    before do
+      stub_const("#{described_class}::ALLOWED_TRACKER_OPTIONS", [:some_option])
+    end
+
+    context 'with an allowed option configured with a static value' do
+      subject { described_class.new(env, { some_option: 'value' }) }
+
+      it 'returns hash with option set' do
+        expect(subject.tracker_options).to eql ({ someOption: 'value' })
+      end
+    end
+
+    context 'with an allowed option configured with a block' do
+      subject { described_class.new(env, { some_option: lambda { |env| return env[:misc] } }) }
+
+      it 'returns hash with option set' do
+        expect(subject.tracker_options).to eql ({ someOption: 'foobar' })
+      end
+    end
+
+    context 'with an allowed option configured with a block returning nil' do
+      subject { described_class.new(env, { some_option: lambda { |env| return env[:non_existing_key] } }) }
+
+      it 'returns an empty hash' do
+        expect(subject.tracker_options).to eql ({})
+      end
+    end
+
+    context 'with a non allowed option' do
+      subject { described_class.new(env, { new_option: 'value' }) }
+
+      it 'returns an empty hash' do
+        expect(subject.tracker_options).to eql ({})
+      end
+    end
+  end
+
   describe "with events" do
     describe "default" do
       def env
@@ -93,6 +134,15 @@ def env
     end
   end
 
+  describe "with user_id tracking" do
+    subject { described_class.new(env, tracker: 'somebody', user_id: lambda { |env| return env[:user_id] } ).render }
+
+    it "will show asyncronous tracker with userId" do
+      expect(subject).to match(%r{ga\('create', 'somebody', {\"userId\":\"123\"}\)})
+      expect(subject).to match(%r{ga\('send', 'pageview'\)})
+    end
+  end
+
   describe "with enhanced_link_attribution" do
     subject { described_class.new(env, tracker: 'happy', enhanced_link_attribution: true).render }