diff --git a/src/FileSystem/FileSystem.php b/src/FileSystem/FileSystem.php
index 5aabd7b..609aa43 100644
--- a/src/FileSystem/FileSystem.php
+++ b/src/FileSystem/FileSystem.php
@@ -9,7 +9,7 @@ final class FileSystem implements FileSystemInterface
     /**
      * {@inheritdoc}
      */
-    public function open($path, $mode)
+    public function open(string $path, string $mode)
     {
         $stream = @fopen($path, $mode);
 
@@ -21,7 +21,7 @@ public function open($path, $mode)
     /**
      * {@inheritdoc}
      */
-    public function getContents($path)
+    public function getContents(string $path): string
     {
         $stream = $this->open($path, 'r');
 
@@ -36,7 +36,7 @@ public function getContents($path)
     /**
      * {@inheritdoc}
      */
-    public function putContents($path, $buffer)
+    public function putContents(string $path, string $buffer): int
     {
         $stream = $this->open($path, 'w');
 
@@ -59,7 +59,7 @@ private function exceptionIfFalse($result)
     /**
      * {@inheritdoc}
      */
-    public function exists($path)
+    public function exists(string $path): bool
     {
         return file_exists($path);
     }
@@ -67,7 +67,7 @@ public function exists($path)
     /**
      * {@inheritdoc}
      */
-    public function isDir($path)
+    public function isDir(string $path): bool
     {
         return is_dir($path);
     }
diff --git a/src/FileSystem/FileSystemInterface.php b/src/FileSystem/FileSystemInterface.php
index 366cd68..c701390 100644
--- a/src/FileSystem/FileSystemInterface.php
+++ b/src/FileSystem/FileSystemInterface.php
@@ -11,7 +11,7 @@ interface FileSystemInterface
      *
      * @return bool true if file exists, false otherwise
      */
-    public function exists($path);
+    public function exists(string $path): bool;
 
     /**
      * Tells whether the filename is a directory
@@ -20,7 +20,7 @@ public function exists($path);
      *
      * @return bool true if the filename exists and is a directory, false otherwise
      */
-    public function isDir($path);
+    public function isDir(string $path): bool;
 
     /**
      * Open a file.
@@ -30,7 +30,7 @@ public function isDir($path);
      *
      * @return resource A stream resource.
      */
-    public function open($path, $mode);
+    public function open(string $path, string $mode);
 
     /**
      * Read the contents of a file.
@@ -39,7 +39,7 @@ public function open($path, $mode);
      *
      * @return string The file contents
      */
-    public function getContents($path);
+    public function getContents(string $path): string;
 
     /**
      * Write a buffer to a file.
@@ -49,5 +49,5 @@ public function getContents($path);
      *
      * @return int The number of bytes written
      */
-    public function putContents($path, $buffer);
+    public function putContents(string $path, string $buffer): int;
 }
diff --git a/src/GPIO.php b/src/GPIO.php
index cbb3558..ab33ac5 100644
--- a/src/GPIO.php
+++ b/src/GPIO.php
@@ -5,6 +5,9 @@
 use PiPHP\GPIO\FileSystem\FileSystem;
 use PiPHP\GPIO\FileSystem\FileSystemInterface;
 use PiPHP\GPIO\Interrupt\InterruptWatcher;
+use PiPHP\GPIO\Interrupt\InterruptWatcherInterface;
+use PiPHP\GPIO\Pin\InputPinInterface;
+use PiPHP\GPIO\Pin\OutputPinInterface;
 use PiPHP\GPIO\Pin\Pin;
 use PiPHP\GPIO\Pin\InputPin;
 use PiPHP\GPIO\Pin\OutputPin;
@@ -29,7 +32,7 @@ public function __construct(FileSystemInterface $fileSystem = null, callable $st
     /**
      * {@inheritdoc}
      */
-    public function getInputPin($number)
+    public function getInputPin(int $number): InputPinInterface
     {
         return new InputPin($this->fileSystem, $number);
     }
@@ -37,7 +40,7 @@ public function getInputPin($number)
     /**
      * {@inheritdoc}
      */
-    public function getOutputPin($number, $exportDirection = Pin::DIRECTION_OUT)
+    public function getOutputPin($number, $exportDirection = Pin::DIRECTION_OUT): OutputPinInterface
     {
         if ($exportDirection !== Pin::DIRECTION_OUT && $exportDirection !== Pin::DIRECTION_LOW && $exportDirection !== Pin::DIRECTION_HIGH) {
             throw new \InvalidArgumentException('exportDirection has to be an OUT type (OUT/LOW/HIGH).');
@@ -49,7 +52,7 @@ public function getOutputPin($number, $exportDirection = Pin::DIRECTION_OUT)
     /**
      * {@inheritdoc}
      */
-    public function createWatcher()
+    public function createWatcher(): InterruptWatcherInterface
     {
         return new InterruptWatcher($this->fileSystem, $this->streamSelect);
     }
diff --git a/src/GPIOInterface.php b/src/GPIOInterface.php
index ac7ac63..737b37a 100644
--- a/src/GPIOInterface.php
+++ b/src/GPIOInterface.php
@@ -15,7 +15,7 @@ interface GPIOInterface
      *
      * @return InputPinInterface
      */
-    public function getInputPin($number);
+    public function getInputPin(int $number): InputPinInterface;
 
     /**
      * Get an output pin.
@@ -24,12 +24,12 @@ public function getInputPin($number);
      *
      * @return OutputPinInterface
      */
-    public function getOutputPin($number);
+    public function getOutputPin(int $number): OutputPinInterface;
 
     /**
      * Create an interrupt watcher.
      *
      * @return InterruptWatcherInterface
      */
-    public function createWatcher();
+    public function createWatcher(): InterruptWatcherInterface;
 }
diff --git a/src/Interrupt/InterruptWatcher.php b/src/Interrupt/InterruptWatcher.php
index 88265ab..d5bb08d 100644
--- a/src/Interrupt/InterruptWatcher.php
+++ b/src/Interrupt/InterruptWatcher.php
@@ -75,7 +75,7 @@ public function unregister(InputPinInterface $pin)
     /**
      * {@inheritdoc}
      */
-    public function watch($timeout)
+    public function watch(int $timeout): bool
     {
         $seconds = floor($timeout / 1000);
         $carry = $timeout - ($seconds * 1000);
diff --git a/src/Interrupt/InterruptWatcherInterface.php b/src/Interrupt/InterruptWatcherInterface.php
index c802934..e3f6931 100644
--- a/src/Interrupt/InterruptWatcherInterface.php
+++ b/src/Interrupt/InterruptWatcherInterface.php
@@ -26,5 +26,5 @@ public function unregister(InputPinInterface $pin);
      *
      * @param int $timeout The maximum time to watch for in milliseconds.
      */
-    public function watch($timeout);
+    public function watch(int $timeout): bool;
 }
diff --git a/src/Pin/InputPin.php b/src/Pin/InputPin.php
index e8d2eee..b046242 100644
--- a/src/Pin/InputPin.php
+++ b/src/Pin/InputPin.php
@@ -24,7 +24,7 @@ public function __construct(FileSystemInterface $fileSystem, $number)
     /**
      * {@inheritdoc}
      */
-    public function getEdge()
+    public function getEdge(): string
     {
         $edgeFile = $this->getPinFile(self::GPIO_PIN_FILE_EDGE);
         return trim($this->fileSystem->getContents($edgeFile));
@@ -33,7 +33,7 @@ public function getEdge()
     /**
      * {@inheritdoc}
      */
-    public function setEdge($edge)
+    public function setEdge(string $edge)
     {
         $edgeFile = $this->getPinFile(self::GPIO_PIN_FILE_EDGE);
         $this->fileSystem->putContents($edgeFile, $edge);
diff --git a/src/Pin/InputPinInterface.php b/src/Pin/InputPinInterface.php
index 49aba9f..c474ecb 100644
--- a/src/Pin/InputPinInterface.php
+++ b/src/Pin/InputPinInterface.php
@@ -14,12 +14,12 @@ interface InputPinInterface extends PinInterface
      *
      * @return string The pin edge value
      */
-    public function getEdge();
+    public function getEdge(): string;
 
     /**
      * Set the pin edge.
      *
      * @param string $edge The pin edge value to set
      */
-    public function setEdge($edge);
+    public function setEdge(string $edge);
 }
diff --git a/src/Pin/OutputPin.php b/src/Pin/OutputPin.php
index 713940d..9ded559 100644
--- a/src/Pin/OutputPin.php
+++ b/src/Pin/OutputPin.php
@@ -28,7 +28,7 @@ public function __construct(FileSystemInterface $fileSystem, $number, $exportDir
     /**
      * {@inheritdoc}
      */
-    public function setValue($value)
+    public function setValue(int $value)
     {
         $valueFile = $this->getPinFile(self::GPIO_PIN_FILE_VALUE);
         $this->fileSystem->putContents($valueFile, $value);
diff --git a/src/Pin/OutputPinInterface.php b/src/Pin/OutputPinInterface.php
index aa1a28f..92fdc48 100644
--- a/src/Pin/OutputPinInterface.php
+++ b/src/Pin/OutputPinInterface.php
@@ -9,5 +9,5 @@ interface OutputPinInterface extends PinInterface
      *
      * @param int $value The value to set
      */
-    public function setValue($value);
+    public function setValue(int $value);
 }
diff --git a/src/Pin/Pin.php b/src/Pin/Pin.php
index c0926e8..993b7a3 100644
--- a/src/Pin/Pin.php
+++ b/src/Pin/Pin.php
@@ -35,7 +35,7 @@ abstract class Pin implements PinInterface
      * @param FileSystemInterface $fileSystem An object that provides file system access
      * @param int                 $number     The number of the pin
      */
-    public function __construct(FileSystemInterface $fileSystem, $number)
+    public function __construct(FileSystemInterface $fileSystem, int $number)
     {
         $this->fileSystem = $fileSystem;
         $this->number = $number;
@@ -46,7 +46,7 @@ public function __construct(FileSystemInterface $fileSystem, $number)
     /**
      * {@inheritdoc}
      */
-    public function getNumber()
+    public function getNumber(): int
     {
         return $this->number;
     }
@@ -79,7 +79,7 @@ public function unexport()
     /**
      * {@inheritdoc}
      */
-    protected function isExported()
+    protected function isExported(): bool
     {
         $directory = $this->getPinDirectory();
 
@@ -89,7 +89,7 @@ protected function isExported()
     /**
      * {@inheritdoc}
      */
-    protected function getDirection()
+    protected function getDirection(): ?string
     {
         $directionFile = $this->getPinFile(self::GPIO_PIN_FILE_DIRECTION);
 
@@ -103,7 +103,7 @@ protected function getDirection()
     /**
      * {@inheritdoc}
      */
-    protected function setDirection($direction)
+    protected function setDirection(string $direction)
     {
         if ($this->getDirection() !== $direction) {
             $directionFile = $this->getPinFile(self::GPIO_PIN_FILE_DIRECTION);
@@ -115,7 +115,7 @@ protected function setDirection($direction)
     /**
      * {@inheritdoc}
      */
-    public function getValue()
+    public function getValue(): int
     {
         $valueFile = $this->getPinFile(self::GPIO_PIN_FILE_VALUE);
         return (int) trim($this->fileSystem->getContents($valueFile));
@@ -128,7 +128,7 @@ public function getValue()
      *
      * @return string The file path
      */
-    private function getFile($file)
+    private function getFile(string $file): string
     {
         return self::GPIO_PATH . $file;
     }
@@ -138,7 +138,7 @@ private function getFile($file)
      *
      * @return string
      */
-    protected function getPinDirectory()
+    protected function getPinDirectory(): string
     {
         return self::GPIO_PATH . self::GPIO_PREFIX . $this->getNumber();
     }
@@ -150,7 +150,7 @@ protected function getPinDirectory()
      *
      * @return string
      */
-    protected function getPinFile($file)
+    protected function getPinFile(string $file): string
     {
         return $this->getPinDirectory() . '/' . $file;
     }
@@ -160,7 +160,7 @@ protected function getPinFile($file)
      *
      * @param string $file The file to write to
      */
-    private function writePinNumberToFile($file)
+    private function writePinNumberToFile(string $file)
     {
         $this->fileSystem->putContents($file, $this->getNumber());
     }
diff --git a/src/Pin/PinInterface.php b/src/Pin/PinInterface.php
index d8c2792..12a6281 100644
--- a/src/Pin/PinInterface.php
+++ b/src/Pin/PinInterface.php
@@ -12,7 +12,7 @@ interface PinInterface
      *
      * @return int
      */
-    public function getNumber();
+    public function getNumber(): int;
 
     /**
      * Export the pin.
@@ -29,5 +29,5 @@ public function unexport();
      *
      * @return int
      */
-    public function getValue();
+    public function getValue(): int;
 }
diff --git a/test/FileSystem/VFS.php b/test/FileSystem/VFS.php
index 57d9ceb..af1758e 100644
--- a/test/FileSystem/VFS.php
+++ b/test/FileSystem/VFS.php
@@ -8,21 +8,22 @@ class VFS implements FileSystemInterface
 {
     private $vfs = [];
 
-    public function open($path, $mode)
+    public function open(string $path, string $mode)
     {
     }
 
-    public function getContents($path)
+    public function getContents(string $path): string
     {
         return $this->vfs[$path];
     }
 
-    public function putContents($path, $buffer)
+    public function putContents(string $path, string $buffer): int
     {
         $this->vfs[$path] = $buffer;
+        return 0;
     }
 
-    public function exists($path)
+    public function exists(string $path): bool
     {
         $regex = sprintf('#^%s(\/.+)?$#', preg_quote($path, '#'));
 
@@ -31,7 +32,7 @@ public function exists($path)
         }));
     }
 
-    public function isDir($path)
+    public function isDir(string $path): bool
     {
         return $this->exists($path) && !array_key_exists($path, $this->vfs);
     }