diff --git a/composer.json b/composer.json
index 3c77c65..00670e5 100644
--- a/composer.json
+++ b/composer.json
@@ -34,7 +34,8 @@
     "require": {
         "amphp/amp": "^2",
         "amphp/byte-stream": "^1",
-        "amphp/parallel": "^1"
+        "amphp/parallel": "^1",
+        "amphp/sync": "^1.0"
     },
     "require-dev": {
         "amphp/phpunit-util": "^1",
@@ -56,5 +57,14 @@
         "platform": {
             "php": "7.0.0"
         }
+    },
+    "scripts": {
+        "check": [
+            "@cs",
+            "@test"
+        ],
+        "cs": "PHP_CS_FIXER_IGNORE_ENV=1 php-cs-fixer fix -v --diff --dry-run",
+        "cs-fix": "PHP_CS_FIXER_IGNORE_ENV=1 php-cs-fixer fix -v --diff",
+        "test": "@php -dzend.assertions=1 -dassert.exception=1 ./vendor/bin/phpunit --coverage-text"
     }
 }
diff --git a/lib/functions.php b/lib/functions.php
index 21eafb4..c9ead27 100644
--- a/lib/functions.php
+++ b/lib/functions.php
@@ -2,8 +2,14 @@
 
 namespace Amp\File;
 
+use Amp\CancellationToken;
+use Amp\Delayed;
 use Amp\Loop;
+use Amp\NullCancellationToken;
 use Amp\Promise;
+use Amp\Sync\Lock;
+
+use function Amp\call;
 
 const LOOP_STATE_IDENTIFIER = Driver::class;
 
@@ -350,3 +356,77 @@ function put(string $path, string $contents): Promise
 {
     return filesystem()->put($path, $contents);
 }
+
+/**
+ * Asynchronously lock a file
+ * Resolves with a callable that MUST eventually be called in order to release the lock.
+ *
+ * @param string            $file    File to lock
+ * @param bool              $shared  Whether to acquire a shared or exclusive lock (\LOCK_SH or \LOCK_EX, see PHP flock docs)
+ * @param integer           $polling Polling interval for lock in milliseconds
+ * @param CancellationToken $token   Cancellation token
+ *
+ * @return \Amp\Promise Resolves with an \Amp\Sync\Lock
+ */
+function lock(string $file, bool $shared, int $polling = 100, CancellationToken $token = null): Promise
+{
+    return call(static function () use ($file, $shared, $polling, $token) {
+        $operation = $shared ? \LOCK_SH : \LOCK_EX;
+        $token = $token ?? new NullCancellationToken;
+        if (!yield exists($file)) {
+            yield \touch($file);
+            StatCache::clear($file);
+        }
+        $operation |= LOCK_NB;
+        $res = \fopen($file, 'c');
+
+        while (!\flock($res, $operation, $wouldblock)) {
+            if (!$wouldblock) {
+                throw new FilesystemException("Failed acquiring lock on file.");
+            }
+            yield new Delayed($polling);
+            $token->throwIfRequested();
+        }
+
+        return new Lock(
+            0,
+            static function () use (&$res) {
+                if ($res) {
+                    \flock($res, LOCK_UN);
+                    \fclose($res);
+                    $res = null;
+                }
+            }
+        );
+    });
+}
+
+/**
+ * Asynchronously lock a file (shared lock)
+ * Resolves with a callable that MUST eventually be called in order to release the lock.
+ *
+ * @param string            $file    File to lock
+ * @param integer           $polling Polling interval for lock in milliseconds
+ * @param CancellationToken $token   Cancellation token
+ *
+ * @return \Amp\Promise Resolves with a callable that MUST eventually be called in order to release the lock.
+ */
+function lockShared(string $file, int $polling = 100, CancellationToken $token = null): Promise
+{
+    return lock($file, true, $polling, $token ?? new NullCancellationToken);
+}
+
+/**
+ * Asynchronously lock a file (exclusive lock)
+ * Resolves with a callable that MUST eventually be called in order to release the lock.
+ *
+ * @param string            $file    File to lock
+ * @param integer           $polling Polling interval for lock in milliseconds
+ * @param CancellationToken $token   Cancellation token
+ *
+ * @return \Amp\Promise Resolves with a callable that MUST eventually be called in order to release the lock.
+ */
+function lockExclusive(string $file, int $polling = 100, CancellationToken $token = null): Promise
+{
+    return lock($file, false, $polling, $token ?? new NullCancellationToken);
+}
diff --git a/test/HandleTest.php b/test/HandleTest.php
index 13cea7d..ed37ee5 100644
--- a/test/HandleTest.php
+++ b/test/HandleTest.php
@@ -3,8 +3,13 @@
 namespace Amp\File\Test;
 
 use Amp\ByteStream\ClosedException;
+use Amp\Delayed;
 use Amp\File;
 use Amp\PHPUnit\TestCase;
+use Amp\Sync\Lock;
+use Amp\TimeoutCancellationToken;
+
+use function Amp\Promise\timeout;
 
 abstract class HandleTest extends TestCase
 {
@@ -232,7 +237,89 @@ public function testMode()
             yield $handle->close();
         });
     }
-
+    /**
+     * Try locking file exclusively.
+     *
+     * @param string $file File
+     * @param int $polling Polling interval
+     * @param int $timeout Lock timeout
+     * @return void
+     */
+    private function tryLockExclusive(string $file, int $polling, int $timeout)
+    {
+        return File\lockExclusive($file, $polling, new TimeoutCancellationToken($timeout));
+    }
+    /**
+     * Try locking file in shared mode.
+     *
+     * @param string $file File
+     * @param int $polling Polling interval
+     * @param int $timeout Lock timeout
+     * @return void
+     */
+    private function tryLockShared(string $file, int $polling, int $timeout)
+    {
+        return File\lockShared($file, $polling, new TimeoutCancellationToken($timeout));
+    }
+    public function testExclusiveLock()
+    {
+        $this->execute(function () {
+            $primary = null;
+            $secondary = null;
+            try {
+                try {
+                    $primary = yield $this->tryLockExclusive(__FILE__, 100, 100);
+                    $this->assertInstanceOf(Lock::class, $primary);
+
+                    $unlocked = false;
+                    $try = $this->tryLockShared(__FILE__, 100, 10000);
+                    $try->onResolve(static function ($e, $secondaryUnlock) use (&$unlocked, &$secondary) {
+                        if ($e) {
+                            throw $e;
+                        }
+                        $unlocked = true;
+                        $secondary = $secondaryUnlock;
+                    });
+
+                    $this->assertFalse($unlocked, "The lock wasn't acquired");
+                } finally {
+                    if ($primary) {
+                        $primary->release();
+                    }
+                }
+
+                yield new Delayed(100 * 2);
+                $this->assertTrue($unlocked, "The lock wasn't released");
+
+                yield $try;
+                $this->assertInstanceOf(Lock::class, $secondary);
+            } finally {
+                if ($secondary) {
+                    $secondary->release();
+                }
+            }
+        });
+    }
+    public function testSharedLock()
+    {
+        $this->execute(function () {
+            $primary = null;
+            $secondary = null;
+            try {
+                $primary = yield $this->tryLockShared(__FILE__, 100, 100);
+                $this->assertInstanceOf(Lock::class, $primary);
+                $secondary = yield $this->tryLockShared(__FILE__, 100, 100);
+                $this->assertInstanceOf(Lock::class, $secondary);
+            } finally {
+                if ($primary) {
+                    $primary->release();
+                }
+                if ($secondary) {
+                    $secondary->release();
+                }
+            }
+        });
+    }
     public function testClose()
     {
         $this->execute(function () {