diff --git a/.rubocop.yml b/.rubocop.yml
index 04b2ef9..352221b 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -16,6 +16,9 @@ AllCops:
 Standard/SemanticBlocks:
   Enabled: false
 
+Style/FrozenStringLiteralComment:
+  Enabled: true
+
 Style/TrailingCommaInArrayLiteral:
   EnforcedStyleForMultiline: no_comma
 
diff --git a/Rakefile b/Rakefile
index 49379a0..3c55aa4 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "bundler/gem_tasks"
 require "rspec/core/rake_task"
 require "rubocop/rake_task"
diff --git a/gemfiles/rails42.gemfile b/gemfiles/rails42.gemfile
index 35f32f7..3f8378a 100644
--- a/gemfiles/rails42.gemfile
+++ b/gemfiles/rails42.gemfile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 source "https://rubygems.org"
 
 gem "activesupport", "~> 4.2"
diff --git a/gemfiles/railsmaster.gemfile b/gemfiles/railsmaster.gemfile
index 3b1e7ba..e75141d 100644
--- a/gemfiles/railsmaster.gemfile
+++ b/gemfiles/railsmaster.gemfile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 source "https://rubygems.org"
 
 gem "rails", github: "rails/rails"
diff --git a/lib/active_delivery.rb b/lib/active_delivery.rb
index 2f7af4a..274f357 100644
--- a/lib/active_delivery.rb
+++ b/lib/active_delivery.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "active_delivery/version"
 require "active_delivery/base"
 require "active_delivery/callbacks" if defined?(ActiveSupport)
diff --git a/lib/active_delivery/lines/base.rb b/lib/active_delivery/lines/base.rb
index ad59065..bbedddb 100644
--- a/lib/active_delivery/lines/base.rb
+++ b/lib/active_delivery/lines/base.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ActiveDelivery
   module Lines
     class Base
diff --git a/lib/active_delivery/lines/mailer.rb b/lib/active_delivery/lines/mailer.rb
index 50e0254..2e5a6b3 100644
--- a/lib/active_delivery/lines/mailer.rb
+++ b/lib/active_delivery/lines/mailer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ActiveDelivery
   module Lines
     class Mailer < Base
diff --git a/lib/active_delivery/testing.rb b/lib/active_delivery/testing.rb
index 3ad3622..e9106d7 100644
--- a/lib/active_delivery/testing.rb
+++ b/lib/active_delivery/testing.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ActiveDelivery
   module TestDelivery
     class << self
diff --git a/lib/active_delivery/testing/rspec.rb b/lib/active_delivery/testing/rspec.rb
index 52646a3..ec36e4a 100644
--- a/lib/active_delivery/testing/rspec.rb
+++ b/lib/active_delivery/testing/rspec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ActiveDelivery
   class HaveDeliveredTo < RSpec::Matchers::BuiltIn::BaseMatcher
     attr_reader :delivery_class, :event, :args, :params, :sync_value
diff --git a/lib/active_delivery/version.rb b/lib/active_delivery/version.rb
index 9c1a472..789eb1f 100644
--- a/lib/active_delivery/version.rb
+++ b/lib/active_delivery/version.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ActiveDelivery
   VERSION = "0.1.1"
 end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index b566ce8..530801a 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 ENV["RACK_ENV"] = "test"
 
 require "bundler/setup"
diff --git a/spec/support/quack.rb b/spec/support/quack.rb
index e753ffb..ae2272b 100644
--- a/spec/support/quack.rb
+++ b/spec/support/quack.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class Quack
   attr_reader :mid