diff --git a/lib/unleash/client.rb b/lib/unleash/client.rb
index 11d078f2..a05213ae 100644
--- a/lib/unleash/client.rb
+++ b/lib/unleash/client.rb
@@ -64,7 +64,10 @@ def register
       http.use_ssl = true if uri.scheme == 'https'
       http.open_timeout = Unleash.configuration.timeout # in seconds
       http.read_timeout = Unleash.configuration.timeout # in seconds
-      headers = {'Content-Type' => 'application/json'}
+
+      headers = (Unleash.configuration.custom_http_headers || {}).dup
+      headers['Content-Type'] = 'application/json'
+
       request = Net::HTTP::Post.new(uri.request_uri, headers)
       request.body = info.to_json
 
diff --git a/lib/unleash/configuration.rb b/lib/unleash/configuration.rb
index 32067b5d..372d01aa 100644
--- a/lib/unleash/configuration.rb
+++ b/lib/unleash/configuration.rb
@@ -4,6 +4,7 @@
 module Unleash
   class Configuration
     attr_accessor :url, :app_name, :instance_id,
+      :custom_http_headers,
       :disable_metrics, :timeout, :retry_limit,
       :refresh_interval, :metrics_interval,
       :backup_file, :logger, :log_level
@@ -13,6 +14,11 @@ def initialize(opts = {})
       self.url           = opts[:url]         || nil
       self.instance_id   = opts[:instance_id] || SecureRandom.uuid
 
+      if opts[:custom_http_headers].is_a?(Hash) || opts[:custom_http_headers].nil?
+        self.custom_http_headers = opts[:custom_http_headers] || {}
+      else
+        raise ArgumentError, "custom_http_headers must be a hash."
+      end
       self.disable_metrics  = opts[:disable_metrics]  || false
       self.refresh_interval = opts[:refresh_interval] || 15
       self.metrics_interval = opts[:metrics_interval] || 10
@@ -42,7 +48,11 @@ def metrics_interval_in_millis
 
     def validate!
       if self.app_name.nil? or self.url.nil?
-        raise ArgumentError, "URL and app_name are required"
+        raise ArgumentError, "URL and app_name are required parameters."
+      end
+
+      if ! self.custom_http_headers.is_a?(Hash)
+        raise ArgumentError, "custom_http_headers must be a hash."
       end
     end
 
diff --git a/lib/unleash/metrics_reporter.rb b/lib/unleash/metrics_reporter.rb
index 175bebe1..8c3b8b7a 100755
--- a/lib/unleash/metrics_reporter.rb
+++ b/lib/unleash/metrics_reporter.rb
@@ -43,7 +43,9 @@ def send
       http.use_ssl = true if uri.scheme == 'https'
       http.open_timeout = Unleash.configuration.timeout # in seconds
       http.read_timeout = Unleash.configuration.timeout # in seconds
-      headers = {'Content-Type' => 'application/json'}
+
+      headers = (Unleash.configuration.custom_http_headers || {}).dup
+      headers['Content-Type'] = 'application/json'
       request = Net::HTTP::Post.new(uri.request_uri, headers)
       request.body = generated_report.to_json
 
diff --git a/lib/unleash/toggle_fetcher.rb b/lib/unleash/toggle_fetcher.rb
index f41aab44..ddc40c02 100755
--- a/lib/unleash/toggle_fetcher.rb
+++ b/lib/unleash/toggle_fetcher.rb
@@ -38,6 +38,7 @@ def toggles
     end
 
     # rename to refresh_from_server!  ??
+    # TODO: should simplify by moving uri / http initialization to class initialization
     def fetch
       Unleash.logger.debug "fetch()"
       Unleash.logger.debug "ETag: #{self.etag}" unless self.etag.nil?
@@ -47,8 +48,12 @@ def fetch
       http.use_ssl = true if uri.scheme == 'https'
       http.open_timeout = Unleash.configuration.timeout # in seconds
       http.read_timeout = Unleash.configuration.timeout # in seconds
-      request = Net::HTTP::Get.new(uri.request_uri)
-      request['If-None-Match'] = self.etag unless self.etag.nil?
+
+      headers = (Unleash.configuration.custom_http_headers || {}).dup
+      headers['Content-Type'] = 'application/json'
+      headers['If-None-Match'] = self.etag unless self.etag.nil?
+
+      request = Net::HTTP::Get.new(uri.request_uri, headers)
 
       response = http.request(request)
 
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 3066dd78..00bf101b 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -2,9 +2,13 @@
 require "unleash"
 require "unleash/client"
 
+require 'webmock/rspec'
+
 require 'coveralls'
 Coveralls.wear!
 
+WebMock.disable_net_connect!(allow_localhost: false)
+
 RSpec.configure do |config|
   # Enable flags like --only-failures and --next-failure
   config.example_status_persistence_file_path = ".rspec_status"
diff --git a/spec/unleash/client_spec.rb b/spec/unleash/client_spec.rb
index 6fe37f0e..d3398ef7 100644
--- a/spec/unleash/client_spec.rb
+++ b/spec/unleash/client_spec.rb
@@ -5,6 +5,62 @@
     expect(Unleash::VERSION).not_to be nil
   end
 
+  it "Uses custom http headers when initializing client" do
+    WebMock.stub_request(:post, "http://test-url//client/register")
+      .with(
+        headers: {
+        'Accept'=>'*/*',
+        'Content-Type'=>'application/json',
+        'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3',
+        'User-Agent'=>'Ruby',
+        'X-Api-Key'=>'123'
+        })
+      .to_return(status: 200, body: "", headers: {})
+    WebMock.stub_request(:post, "http://test-url//client/metrics").
+       with(
+         headers: {
+        'Accept'=>'*/*',
+        'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3',
+        'Content-Type'=>'application/json',
+        'User-Agent'=>'Ruby'
+         }).
+       to_return(status: 200, body: "", headers: {})
+
+
+    Unleash.configure do |config|
+      config.url      = 'http://test-url/'
+      config.app_name = 'my-test-app'
+      config.instance_id = 'rspec/test'
+      config.custom_http_headers = {'X-API-KEY' => '123'}
+    end
+
+    unleash_client = Unleash::Client.new(
+      url: 'http://test-url/',
+      app_name: 'my-test-app',
+      instance_id: 'rspec/test',
+      custom_http_headers: {'X-API-KEY' => '123'}
+    )
+
+    expect(
+      a_request(:post, "http://test-url//client/register")
+      .with( headers: {'Content-Type': 'application/json'})
+      .with( headers: {'X-API-KEY': '123', 'Content-Type': 'application/json'})
+    ).to have_been_made.once
+
+    expect(
+      a_request(:get, "http://test-url//client/features")
+      .with( headers: {'X-API-KEY': '123'})
+    ).to have_been_made.once
+
+    # Test now sending of metrics
+    Unleash.reporter.send
+    expect(
+      a_request(:post, "http://test-url//client/metrics")
+      .with( headers: {'Content-Type': 'application/json'})
+      .with( headers: {'X-API-KEY': '123', 'Content-Type': 'application/json'})
+    ).to have_been_made.once
+  end
+
   it "does something useful" do
     expect(false).to eq(false)
   end
diff --git a/spec/unleash/configuration_spec.rb b/spec/unleash/configuration_spec.rb
index 14f5f509..02ac0fbc 100644
--- a/spec/unleash/configuration_spec.rb
+++ b/spec/unleash/configuration_spec.rb
@@ -4,12 +4,17 @@
 RSpec.describe Unleash do
 
   describe 'Configuration' do
+    before do
+      Unleash.configuration = nil
+    end
+
     it "should have the correct defaults" do
-      config = Unleash::Configuration.new()
+      config = Unleash::Configuration.new
 
       expect(config.app_name).to be_nil
       expect(config.url).to be_nil
       expect(config.instance_id).to be_truthy
+      expect(config.custom_http_headers).to eq({})
       expect(config.disable_metrics).to be_falsey
 
       expect(config.refresh_interval).to eq(15)
@@ -17,12 +22,13 @@
       expect(config.timeout).to eq(30)
       expect(config.retry_limit).to eq(1)
 
-      # expect(config.validate!).to raise_error(ArgumentError)
       expect(config.backup_file).to_not be_nil
+
+      expect{ config.validate! }.to raise_error(ArgumentError)
     end
 
     it "should by default be invalid" do
-      config = Unleash::Configuration.new()
+      config = Unleash::Configuration.new
       expect{ config.validate! }.to raise_error(ArgumentError)
     end
 
@@ -50,6 +56,53 @@
       expect(config.client_register_url).to eq('https://testurl/api/client/register')
     end
 
-  end
+    it "should allow hashes for custom_http_headers via yield" do
+      Unleash.configure do |config|
+        config.url      = 'http://test-url/'
+        config.app_name = 'my-test-app'
+        config.custom_http_headers = {'X-API-KEY': '123'}
+      end
+      expect{ Unleash.configuration.validate! }.not_to raise_error
+      expect(Unleash.configuration.custom_http_headers).to eq({'X-API-KEY': '123'})
+    end
+
+    it "should allow hashes for custom_http_headers via new client" do
+      config = Unleash::Configuration.new(
+        url: 'https://testurl/api',
+        app_name: 'test-app',
+        custom_http_headers: {'X-API-KEY': '123'})
+
+      expect{ config.validate! }.not_to raise_error
+      expect(config.custom_http_headers).to eq({'X-API-KEY': '123'})
+    end
 
-end
\ No newline at end of file
+    it "should not accept invalid custom_http_headers via yield" do
+      expect do
+        Unleash.configure do |config|
+          config.url      = 'http://test-url/'
+          config.app_name = 'my-test-app'
+          config.custom_http_headers = 123.456
+        end
+      end.to raise_error(ArgumentError)
+    end
+
+    it "should not accept invalid custom_http_headers via new client" do
+      WebMock.stub_request(:post, "http://test-url//client/register").
+        with(
+          headers: {
+          'Accept'=>'*/*',
+          'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3',
+          'Content-Type'=>'application/json',
+          'User-Agent'=>'Ruby'
+          }).
+        to_return(status: 200, body: "", headers: {})
+
+      expect{ Unleash::Client.new(
+        url: 'https://testurl/api',
+        app_name: 'test-app',
+        custom_http_headers: 123.0,
+        disable_metrics: true
+      ) }.to raise_error(ArgumentError)
+    end
+  end
+end
diff --git a/unleash-client.gemspec b/unleash-client.gemspec
index cb24275a..cc2a197b 100644
--- a/unleash-client.gemspec
+++ b/unleash-client.gemspec
@@ -28,6 +28,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", "~> 1.14"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "rspec", "~> 3.0"
+  spec.add_development_dependency "webmock", "~> 3.0"
   spec.add_development_dependency "coveralls"
 
 end