diff --git a/src/CompositeStream.php b/src/CompositeStream.php
index 47f057c..084385b 100644
--- a/src/CompositeStream.php
+++ b/src/CompositeStream.php
@@ -4,7 +4,7 @@
 
 use Evenement\EventEmitter;
 
-class CompositeStream extends EventEmitter implements DuplexStreamInterface
+final class CompositeStream extends EventEmitter implements DuplexStreamInterface
 {
     protected $readable;
     protected $writable;
diff --git a/src/DuplexResourceStream.php b/src/DuplexResourceStream.php
index 113c9aa..7869eda 100644
--- a/src/DuplexResourceStream.php
+++ b/src/DuplexResourceStream.php
@@ -6,7 +6,7 @@
 use React\EventLoop\LoopInterface;
 use InvalidArgumentException;
 
-class DuplexResourceStream extends EventEmitter implements DuplexStreamInterface
+final class DuplexResourceStream extends EventEmitter implements DuplexStreamInterface
 {
     private $stream;
     private $loop;
diff --git a/src/ReadableResourceStream.php b/src/ReadableResourceStream.php
index 3edf80e..6a9cd65 100644
--- a/src/ReadableResourceStream.php
+++ b/src/ReadableResourceStream.php
@@ -6,7 +6,7 @@
 use React\EventLoop\LoopInterface;
 use InvalidArgumentException;
 
-class ReadableResourceStream extends EventEmitter implements ReadableStreamInterface
+final class ReadableResourceStream extends EventEmitter implements ReadableStreamInterface
 {
     /**
      * @var resource
diff --git a/src/ThroughStream.php b/src/ThroughStream.php
index 9a22b2d..da2fbb0 100644
--- a/src/ThroughStream.php
+++ b/src/ThroughStream.php
@@ -73,7 +73,7 @@
  * @see DuplexStreamInterface::close()
  * @see WritableStreamInterface::pipe()
  */
-class ThroughStream extends EventEmitter implements DuplexStreamInterface
+final class ThroughStream extends EventEmitter implements DuplexStreamInterface
 {
     private $readable = true;
     private $writable = true;
diff --git a/src/Util.php b/src/Util.php
index d59728e..14ddcfc 100644
--- a/src/Util.php
+++ b/src/Util.php
@@ -2,7 +2,7 @@
 
 namespace React\Stream;
 
-class Util
+final class Util
 {
     /**
      * Pipes all the data from the given $source into the $dest
diff --git a/src/WritableResourceStream.php b/src/WritableResourceStream.php
index 47883e2..a52bc2e 100644
--- a/src/WritableResourceStream.php
+++ b/src/WritableResourceStream.php
@@ -5,7 +5,7 @@
 use Evenement\EventEmitter;
 use React\EventLoop\LoopInterface;
 
-class WritableResourceStream extends EventEmitter implements WritableStreamInterface
+final class WritableResourceStream extends EventEmitter implements WritableStreamInterface
 {
     private $stream;
     private $loop;