diff --git a/README.md b/README.md
index 222b7ec..5f1a329 100644
--- a/README.md
+++ b/README.md
@@ -26,6 +26,8 @@ into arbitrary code.
 - ConsumerInitializer = ClassName | Consumer
 - ServiceInitializer = ConsumerInitializer | Value
 
+Construction
+------------
 Container is initialized with raw values, no Initalizers allowed:
 
 ```php
@@ -40,6 +42,8 @@ You can set a name which will be used to refer to container itself:
 $container = new SD\DependencyInjection\Container([], 'container');
 ```
 
+Registering services
+--------------------
 Services are registered with ServiceInitializers:
 
 ```php
@@ -62,6 +66,19 @@ $container->register('helloWorld', function ($name) {
 $container->register('name', $container->value('Skywalker'));
 ```
 
+You can extend registered services:
+
+```php
+$container->register('currency', SD\Currency\Repository::class);
+$container->extend('currency', function ($container, $currency) {
+    $store = $container->produce(SD\CurrencyStore\Wpdb::class);
+    $currency->setStore($store);
+    return $currency;
+});
+```
+
+Consumer production
+-------------------
 Consumers are produced with ConsumerInitializers (Value is not supported):
 
 ```php
diff --git a/src/Container.php b/src/Container.php
index b1e87d0..678d2e2 100644
--- a/src/Container.php
+++ b/src/Container.php
@@ -21,6 +21,20 @@ public function register($name, $initializer) {
         }
     }
 
+    public function extend($name, $extender) {
+        if (isset($this->services[$name])) {
+            $this->services[$name] = $this->produce($extender);
+        } elseif (isset($this->initializers[$name])) {
+            $initializer = $this->initializers[$name];
+            $this->initializers[$name] = function () use ($extender, $initializer, $name) {
+                $this->services[$name] = $this->produce($initializer);
+                return $this->inject($extender);
+            };
+        } else {
+            throw new Exception("Cannot extend unknown service $name");
+        }
+    }
+
     public function value($value) {
         return new Value($value);
     }
diff --git a/tests/ContainerTest.php b/tests/ContainerTest.php
index 79558a6..c2bb2df 100644
--- a/tests/ContainerTest.php
+++ b/tests/ContainerTest.php
@@ -14,7 +14,7 @@ public function testRegisterClassName() {
             ],
             'container'
         );
-        $serviceName = 'hello_world';
+        $serviceName = 'helloWorld';
         $className = HelloWorldService::class;
         $container->register($serviceName, $className);
         $service = $container->get($serviceName);
@@ -30,4 +30,23 @@ public function testDetectCyclicDependencies() {
         $this->expectException(Exception::class);
         $container->get('a');
     }
+
+    public function testExtend() {
+        $name1 = 'Jar Jar Binks';
+        $name2 = 'Palpatine';
+        $container = new Container(
+            [
+                'name' => $name1,
+            ],
+            'container'
+        );
+        $serviceName = 'helloWorld';
+        $container->register($serviceName, HelloWorldService::class);
+        $container->extend($serviceName, function ($helloWorld) use ($name2) {
+            $helloWorld->setName($name2);
+            return $helloWorld;
+        });
+        $service = $container->get($serviceName);
+        $this->assertEquals($name2, $service->getName(), 'Must return modified name');
+    }
 }
diff --git a/tests/HelloWorldService.php b/tests/HelloWorldService.php
index 9e03868..1d76ef3 100644
--- a/tests/HelloWorldService.php
+++ b/tests/HelloWorldService.php
@@ -9,7 +9,7 @@ class HelloWorldService implements DeclarerInterface {
 
     private $name;
 
-    public function __construct($name) {
+    public function __construct(string $name) {
         $this->name = $name;
     }
 
@@ -17,7 +17,11 @@ public function declareDependencies() {
         return ['container'];
     }
 
-    public function getName() {
+    public function setName(string $name) {
+        $this->name = $name;
+    }
+
+    public function getName(): string {
         return $this->name;
     }