diff --git a/lib/Process.php b/lib/Process.php
index 3cb1a35..c730330 100644
--- a/lib/Process.php
+++ b/lib/Process.php
@@ -11,7 +11,7 @@ use Amp\Process\Internal\Windows\Runner as WindowsProcessRunner;
 use Amp\Promise;
 use function Amp\call;
 
-class Process
+final class Process
 {
     /** @var ProcessRunner */
     private $processRunner;
diff --git a/lib/ProcessInputStream.php b/lib/ProcessInputStream.php
index d6e473b..fb89817 100644
--- a/lib/ProcessInputStream.php
+++ b/lib/ProcessInputStream.php
@@ -11,7 +11,7 @@ use Amp\Failure;
 use Amp\Promise;
 use Amp\Success;
 
-class ProcessInputStream implements InputStream
+final class ProcessInputStream implements InputStream
 {
     /** @var Deferred */
     private $initialRead;
diff --git a/lib/ProcessOutputStream.php b/lib/ProcessOutputStream.php
index a86abe4..d80ed70 100644
--- a/lib/ProcessOutputStream.php
+++ b/lib/ProcessOutputStream.php
@@ -10,7 +10,7 @@ use Amp\Deferred;
 use Amp\Failure;
 use Amp\Promise;
 
-class ProcessOutputStream implements OutputStream
+final class ProcessOutputStream implements OutputStream
 {
     /** @var \SplQueue */
     private $queuedWrites;