diff --git a/lib/puma/binder.rb b/lib/puma/binder.rb
index bcaeeec0..7fc59d09 100644
--- a/lib/puma/binder.rb
+++ b/lib/puma/binder.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'uri'
 require 'socket'
 
diff --git a/lib/puma/cli.rb b/lib/puma/cli.rb
index cff013c2..aad89648 100644
--- a/lib/puma/cli.rb
+++ b/lib/puma/cli.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'optparse'
 require 'uri'
 
diff --git a/lib/puma/client.rb b/lib/puma/client.rb
index c395555c..b6087145 100644
--- a/lib/puma/client.rb
+++ b/lib/puma/client.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class IO
   # We need to use this for a jruby work around on both 1.8 and 1.9.
   # So this either creates the constant (on 1.8), or harmlessly
diff --git a/lib/puma/cluster.rb b/lib/puma/cluster.rb
index 3a1aa586..aef6c005 100644
--- a/lib/puma/cluster.rb
+++ b/lib/puma/cluster.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'puma/runner'
 require 'puma/util'
 require 'puma/plugin'
diff --git a/lib/puma/commonlogger.rb b/lib/puma/commonlogger.rb
index ee1f9331..25989e72 100644
--- a/lib/puma/commonlogger.rb
+++ b/lib/puma/commonlogger.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Puma
   # Rack::CommonLogger forwards every request to the given +app+, and
   # logs a line in the
diff --git a/lib/puma/configuration.rb b/lib/puma/configuration.rb
index 7226f7dc..0b2191fa 100644
--- a/lib/puma/configuration.rb
+++ b/lib/puma/configuration.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'puma/rack/builder'
 require 'puma/plugin'
 require 'puma/const'
diff --git a/lib/puma/const.rb b/lib/puma/const.rb
index f9e0a2a2..3e6a7c08 100644
--- a/lib/puma/const.rb
+++ b/lib/puma/const.rb
@@ -1,4 +1,6 @@
 #encoding: utf-8
+# frozen_string_literal: true
+
 module Puma
   class UnsupportedOption < RuntimeError
   end
diff --git a/lib/puma/control_cli.rb b/lib/puma/control_cli.rb
index 692ac820..1a74008e 100644
--- a/lib/puma/control_cli.rb
+++ b/lib/puma/control_cli.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'optparse'
 require 'puma/state_file'
 require 'puma/const'
diff --git a/lib/puma/convenient.rb b/lib/puma/convenient.rb
index 0793c11e..f1d01f30 100644
--- a/lib/puma/convenient.rb
+++ b/lib/puma/convenient.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'puma/launcher'
 require 'puma/configuration'
 
diff --git a/lib/puma/daemon_ext.rb b/lib/puma/daemon_ext.rb
index 56d9e5a2..859a0302 100644
--- a/lib/puma/daemon_ext.rb
+++ b/lib/puma/daemon_ext.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Process
 
   # This overrides the default version because it is broken if it
diff --git a/lib/puma/delegation.rb b/lib/puma/delegation.rb
index e11df31a..c2fdaba5 100644
--- a/lib/puma/delegation.rb
+++ b/lib/puma/delegation.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Puma
   module Delegation
     def forward(what, who)
diff --git a/lib/puma/detect.rb b/lib/puma/detect.rb
index b6e67dc9..0c87a45c 100644
--- a/lib/puma/detect.rb
+++ b/lib/puma/detect.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Puma
   IS_JRUBY = defined?(JRUBY_VERSION)
 
diff --git a/lib/puma/dsl.rb b/lib/puma/dsl.rb
index 220c4f73..0740885d 100644
--- a/lib/puma/dsl.rb
+++ b/lib/puma/dsl.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Puma
   # The methods that are available for use inside the config file.
   # These same methods are used in Puma cli and the rack handler
diff --git a/lib/puma/events.rb b/lib/puma/events.rb
index 10b80270..721cb64a 100644
--- a/lib/puma/events.rb
+++ b/lib/puma/events.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'puma/const'
 require "puma/null_io"
 require 'stringio'
diff --git a/lib/puma/io_buffer.rb b/lib/puma/io_buffer.rb
index 794f5240..5ceb8679 100644
--- a/lib/puma/io_buffer.rb
+++ b/lib/puma/io_buffer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'puma/detect'
 
 if Puma.jruby?
diff --git a/lib/puma/java_io_buffer.rb b/lib/puma/java_io_buffer.rb
index d6434d7a..8a196924 100644
--- a/lib/puma/java_io_buffer.rb
+++ b/lib/puma/java_io_buffer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'java'
 
 # Conservative native JRuby/Java implementation of IOBuffer
diff --git a/lib/puma/jruby_restart.rb b/lib/puma/jruby_restart.rb
index da944e8c..c199ac9f 100644
--- a/lib/puma/jruby_restart.rb
+++ b/lib/puma/jruby_restart.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'ffi'
 
 module Puma
diff --git a/lib/puma/launcher.rb b/lib/puma/launcher.rb
index c6ee7b77..a80d090d 100644
--- a/lib/puma/launcher.rb
+++ b/lib/puma/launcher.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'puma/events'
 require 'puma/detect'
 
diff --git a/lib/puma/minissl.rb b/lib/puma/minissl.rb
index ab1eb324..7dde5239 100644
--- a/lib/puma/minissl.rb
+++ b/lib/puma/minissl.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 begin
   require 'io/wait'
   rescue LoadError
diff --git a/lib/puma/null_io.rb b/lib/puma/null_io.rb
index 80466083..62661fbf 100644
--- a/lib/puma/null_io.rb
+++ b/lib/puma/null_io.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Puma
   # Provides an IO-like object that always appears to contain no data.
   # Used as the value for rack.input when the request has no body.
diff --git a/lib/puma/plugin.rb b/lib/puma/plugin.rb
index 63dc2396..571cff09 100644
--- a/lib/puma/plugin.rb
+++ b/lib/puma/plugin.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Puma
   class UnknownPlugin < RuntimeError; end
 
diff --git a/lib/puma/reactor.rb b/lib/puma/reactor.rb
index 7d0c139d..98a7e824 100644
--- a/lib/puma/reactor.rb
+++ b/lib/puma/reactor.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'puma/util'
 require 'puma/minissl'
 
diff --git a/lib/puma/runner.rb b/lib/puma/runner.rb
index 240cf3b8..b3626645 100644
--- a/lib/puma/runner.rb
+++ b/lib/puma/runner.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'puma/server'
 require 'puma/const'
 
diff --git a/lib/puma/server.rb b/lib/puma/server.rb
index e2e862fd..c8ef87c4 100644
--- a/lib/puma/server.rb
+++ b/lib/puma/server.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'stringio'
 
 require 'puma/thread_pool'
diff --git a/lib/puma/single.rb b/lib/puma/single.rb
index ca07ff63..7a24ef18 100644
--- a/lib/puma/single.rb
+++ b/lib/puma/single.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'puma/runner'
 require 'puma/detect'
 require 'puma/plugin'
diff --git a/lib/puma/state_file.rb b/lib/puma/state_file.rb
index caea14e2..e49f7e5a 100644
--- a/lib/puma/state_file.rb
+++ b/lib/puma/state_file.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'yaml'
 
 module Puma
diff --git a/lib/puma/tcp_logger.rb b/lib/puma/tcp_logger.rb
index a6287693..bbc6dd33 100644
--- a/lib/puma/tcp_logger.rb
+++ b/lib/puma/tcp_logger.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Puma
   class TCPLogger
     def initialize(logger, app, quiet=false)
diff --git a/lib/puma/thread_pool.rb b/lib/puma/thread_pool.rb
index 7b2de557..ed5d87ff 100644
--- a/lib/puma/thread_pool.rb
+++ b/lib/puma/thread_pool.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'thread'
 
 module Puma
diff --git a/lib/puma/util.rb b/lib/puma/util.rb
index 9937a887..31cff15f 100644
--- a/lib/puma/util.rb
+++ b/lib/puma/util.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 major, minor, patch = RUBY_VERSION.split('.').map { |v| v.to_i }
 
 if major == 1 && minor == 9 && patch == 3 && RUBY_PATCHLEVEL < 125