diff --git a/lib/mws.rb b/lib/mws.rb
index 06e9ade..135b2ce 100644
--- a/lib/mws.rb
+++ b/lib/mws.rb
@@ -24,30 +24,17 @@
 require_relative 'mws/api/merchant_fulfillment'
 
 module MWS
-  @@aws_access_key_id = nil
-  @@aws_secret_access_key = nil
-
-  def self.aws_access_key_id=(key_id)
-    @@aws_access_key_id = key_id
-  end
-
-  def self.aws_access_key_id
-    @@aws_access_key_id
-  end
-
-  def self.aws_secret_access_key=(secret_key)
-    @@aws_secret_access_key = secret_key
-  end
-
-  def self.aws_secret_access_key
-    @@aws_secret_access_key
-  end
-
-  def self.new(options = {})
-    @connection = MWS::Connection.new(options)
-  end
-
-  def self.config
-    yield self
+  class << self
+    attr_accessor :aws_access_key_id,
+                  :aws_secret_access_key,
+                  :user_agent
+
+    def config
+      yield self
+    end
+
+    def new(options = {})
+      MWS::Connection.new(options)
+    end
   end
 end
diff --git a/lib/mws/api/base.rb b/lib/mws/api/base.rb
index aa2343c..a5dccf2 100644
--- a/lib/mws/api/base.rb
+++ b/lib/mws/api/base.rb
@@ -6,6 +6,7 @@ class Base
 
       # TODO: Temporary solution, move to configuration
       DEFAULT_TIMEOUT = 2000
+      USER_AGENT = 'User-Agent'.freeze
 
       def initialize(connection)
         @verb ||= :get
@@ -20,7 +21,10 @@ def call(action, params = {})
         when 'GET'
           HTTParty.get(query.request_uri, http_request_options)
         when 'POST'
-          HTTParty.post(query.request_uri, (params[:request_params] || {}).merge(http_request_options))
+          HTTParty.post(
+            query.request_uri,
+            params.fetch(:request_params, {}).deep_merge(http_request_options)
+          )
         end
       end
 
@@ -46,9 +50,11 @@ def method_missing(name, *args)
       end
 
       def http_request_options
-        {
-          timeout: DEFAULT_TIMEOUT
-        }
+        @http_request_options ||= begin
+          options = { timeout: DEFAULT_TIMEOUT }
+          options.merge!(headers: { USER_AGENT => MWS.user_agent }) if MWS.user_agent.present?
+          options
+        end
       end
     end
   end
diff --git a/spec/mws-rb/api/base_spec.rb b/spec/mws-rb/api/base_spec.rb
index 3c1150c..f1846ca 100644
--- a/spec/mws-rb/api/base_spec.rb
+++ b/spec/mws-rb/api/base_spec.rb
@@ -1,6 +1,16 @@
 require 'spec_helper'
 
 describe MWS::API::Base do
+  class TestApi < MWS::API::Base
+    ACTIONS = [:test_action].freeze
+
+    def initialize(connection)
+      @uri = '/Products/2011-10-01'
+      @version = '2011-10-01'
+      super(connection)
+    end
+  end
+
   let(:connection) do
     MWS::Connection.new(aws_access_key_id: 'access key',
                         aws_secret_access_key: 'secret key',
@@ -8,6 +18,7 @@
                         mws_auth_token: 'auth token')
   end
 
+  let(:test_api) { TestApi.new(connection) }
   let(:base) { MWS::API::Base.new(connection) }
 
   it 'should receive a connection object' do
@@ -29,20 +40,61 @@
   end
 
   describe 'method_missing to call actions' do
-    class TestApi < MWS::API::Base
-      ACTIONS = [:test_action].freeze
-      def initialize(connection)
-        @uri = '/Products/2011-10-01'
-        @version = '2011-10-01'
-        super(connection)
-      end
-    end
-
-    let(:test_api) { TestApi.new(connection) }
     before(:each) { allow(HTTParty).to receive(:get).and_return({}) }
 
     it 'should raise exception if Actions do not contain the action name' do
       expect { test_api.action_not_found }.to raise_error(NoMethodError)
     end
   end
+
+  context 'user agent' do
+    subject { test_api.test_action(params) }
+
+    before do |example|
+      @original_user_agent = MWS.user_agent
+      MWS.config { |config| config.user_agent = user_agent }
+    end
+
+    after do
+      subject
+      MWS.config { |config| config.user_agent = @original_user_agent }
+    end
+
+    let(:params) { {} }
+
+    context 'presents in configuration' do
+      let(:user_agent) { 'My Seller Tool/2.0 (Language=Java/1.6.0.11; Platform=Windows/XP' }
+
+      it 'User-Agent is passed' do
+        expect(HTTParty).to receive(:get) do |_uri, params|
+          expect(params[:headers]['User-Agent']).to eq user_agent
+        end
+      end
+
+      context 'other header params presents as well' do
+        let(:params) { { request_params: { headers: { 'Content-Type' => 'text/xml' } } } }
+
+        before { test_api.instance_variable_set(:@verb, :post) }
+
+        it 'merges all header params' do
+          expect(HTTParty).to receive(:post) do |_uri, params|
+            params.fetch(:headers).tap do |headers|
+              expect(headers.fetch('User-Agent')).to eq user_agent
+              expect(headers.fetch('Content-Type')).to eq 'text/xml'
+            end
+          end
+        end
+      end
+    end
+
+    context 'is not present in configuration' do
+      let(:user_agent) { nil }
+
+      it 'headers are not passed' do
+        expect(HTTParty).to receive(:get) do |_uri, params|
+          expect(params.key?(:headers)).to be false
+        end
+      end
+    end
+  end
 end