diff --git a/src/CancellationQueue.php b/src/CancellationQueue.php
index a366994d..f1dd507e 100644
--- a/src/CancellationQueue.php
+++ b/src/CancellationQueue.php
@@ -2,7 +2,7 @@
 
 namespace React\Promise;
 
-class CancellationQueue
+final class CancellationQueue
 {
     private $started = false;
     private $queue = [];
diff --git a/src/Deferred.php b/src/Deferred.php
index 96b93e61..dcd90427 100644
--- a/src/Deferred.php
+++ b/src/Deferred.php
@@ -2,7 +2,7 @@
 
 namespace React\Promise;
 
-class Deferred implements PromisorInterface
+final class Deferred implements PromisorInterface
 {
     private $promise;
     private $resolveCallback;
diff --git a/src/FulfilledPromise.php b/src/FulfilledPromise.php
index 974023d6..b28afd82 100644
--- a/src/FulfilledPromise.php
+++ b/src/FulfilledPromise.php
@@ -2,7 +2,7 @@
 
 namespace React\Promise;
 
-class FulfilledPromise implements PromiseInterface
+final class FulfilledPromise implements PromiseInterface
 {
     private $value;
 
diff --git a/src/LazyPromise.php b/src/LazyPromise.php
index 7a32e27c..678420f5 100644
--- a/src/LazyPromise.php
+++ b/src/LazyPromise.php
@@ -2,7 +2,7 @@
 
 namespace React\Promise;
 
-class LazyPromise implements PromiseInterface
+final class LazyPromise implements PromiseInterface
 {
     private $factory;
     private $promise;
diff --git a/src/Promise.php b/src/Promise.php
index d24ecaa5..7431d2ed 100644
--- a/src/Promise.php
+++ b/src/Promise.php
@@ -2,7 +2,7 @@
 
 namespace React\Promise;
 
-class Promise implements PromiseInterface
+final class Promise implements PromiseInterface
 {
     private $canceller;
     private $result;
diff --git a/src/Queue/SynchronousQueue.php b/src/Queue/SynchronousQueue.php
index ec7a3d36..c20e57e3 100644
--- a/src/Queue/SynchronousQueue.php
+++ b/src/Queue/SynchronousQueue.php
@@ -2,7 +2,7 @@
 
 namespace React\Promise\Queue;
 
-class SynchronousQueue implements QueueInterface
+final class SynchronousQueue implements QueueInterface
 {
     private $queue = [];
 
diff --git a/src/RejectedPromise.php b/src/RejectedPromise.php
index 1e7f6284..848ac335 100644
--- a/src/RejectedPromise.php
+++ b/src/RejectedPromise.php
@@ -2,7 +2,7 @@
 
 namespace React\Promise;
 
-class RejectedPromise implements PromiseInterface
+final class RejectedPromise implements PromiseInterface
 {
     private $reason;