diff --git a/lib/Helpers/MetarHTTPClient.php b/lib/Helpers/MetarHTTPClient.php
index 0624401..956df6a 100644
--- a/lib/Helpers/MetarHTTPClient.php
+++ b/lib/Helpers/MetarHTTPClient.php
@@ -1,8 +1,10 @@
 <?php
+
+declare(strict_types=1);
+
 namespace Ballen\Metar\Helpers;
 
 use GuzzleHttp\Client as HttpClient;
-use \Exception;
 
 /**
  * Metar
@@ -29,7 +31,7 @@ class MetarHTTPClient
      * HTTP Client
      * @param array $config Optional Guzzle configuration.
      */
-    public function __construct($config = [])
+    public function __construct(array $config = [])
     {
         $this->guzzleConf = $config;
     }
@@ -37,9 +39,9 @@ public function __construct($config = [])
     /**
      * Make a HTTP request and retrieve the body.
      * @param string $url The URL to request
-     * @return \Psr\Http\Message\StreamInterface
+     * @throws \GuzzleHttp\Exception\GuzzleException
      */
-    public function getMetarAPIResponse($url)
+    public function getMetarAPIResponse(string $url): string
     {
         $client = new HttpClient($this->guzzleConf);
         $response = $client->get($url);
diff --git a/lib/Metar.php b/lib/Metar.php
index e96d221..e036e18 100644
--- a/lib/Metar.php
+++ b/lib/Metar.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 namespace Ballen\Metar;
 
 /**
@@ -19,7 +22,7 @@ class Metar
     /**
      * Stores the default namespace for loading new METAR providers from.
      */
-    const DEFAULT_PROVIDER = 'Ballen\Metar\Providers\Noaa';
+    public const DEFAULT_PROVIDER = 'Ballen\Metar\Providers\Noaa';
 
     /**
      * Stores the requested airfield/port ICAO code.
@@ -29,7 +32,7 @@ class Metar
 
     /**
      * The METAR string retrieved from the web service.
-     * @var string The raw METAR string retrieved from the web service. 
+     * @var string The raw METAR string retrieved from the web service.
      */
     private $metar;
 
@@ -43,7 +46,7 @@ class Metar
      * Initiates a new METAR object.
      * @param string $icao The airfeild/airport ICAO code.
      */
-    public function __construct($icao)
+    public function __construct(string $icao)
     {
 
         // We force the format of the ICAO code to be upper case!
@@ -73,9 +76,9 @@ public function __toString()
      * @param string $icao ICAO code, eg. EGSS
      * @throws \InvalidArgumentException
      */
-    private function validateIcao($icao)
+    private function validateIcao(string $icao): void
     {
-        if (strlen($icao) != 4) {
+        if (strlen($icao) !== 4) {
             throw new \InvalidArgumentException('ICAO code does not appear to be a valid format');
         }
     }
@@ -84,7 +87,7 @@ private function validateIcao($icao)
      * Changes the default 'NOAA' METAR service provider to another one eg. 'VATSIM'.
      * @param string $provider METAR Provider Class/File name.
      */
-    public function setProvider($provider = self::DEFAULT_PROVIDER)
+    public function setProvider(string $provider = self::DEFAULT_PROVIDER): self
     {
         if (!class_exists($provider)) {
             throw new \InvalidArgumentException('The service provider your specified does not exist in the namespace \'' . $provider . '\'');
diff --git a/lib/Providers/Ivao.php b/lib/Providers/Ivao.php
index b81422f..0177bcc 100644
--- a/lib/Providers/Ivao.php
+++ b/lib/Providers/Ivao.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 namespace Ballen\Metar\Providers;
 
 /**
@@ -21,23 +24,21 @@
  */
 class Ivao extends MetarHTTPClient implements MetarProviderInterface
 {
-
     private $serviceUrl = 'http://wx.ivao.aero/metar.php/?id={{_ICAO_}}';
     private $icao;
 
-    public function __construct($icao)
+    public function __construct(string $icao)
     {
         $this->icao = $icao;
     }
 
-    private function getMetarDataString()
+    /**  @throws \GuzzleHttp\Exception\GuzzleException */
+    private function getMetarDataString(): string
     {
-
-        $data = $this->getMetarAPIResponse(str_replace('{{_ICAO_}}', $this->icao, $this->serviceUrl));
-        return $data;
+        return $this->getMetarAPIResponse(str_replace('{{_ICAO_}}', $this->icao, $this->serviceUrl));
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->getMetarDataString();
     }
diff --git a/lib/Providers/MetarProviderInterface.php b/lib/Providers/MetarProviderInterface.php
index ce221e2..03971d9 100644
--- a/lib/Providers/MetarProviderInterface.php
+++ b/lib/Providers/MetarProviderInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 namespace Ballen\Metar\Providers;
 
 /**
@@ -15,8 +18,7 @@
  */
 interface MetarProviderInterface
 {
+    public function __construct(string $icao);
 
-    function __construct($icao);
-
-    function __toString();
+    public function __toString();
 }
diff --git a/lib/Providers/Noaa.php b/lib/Providers/Noaa.php
index fe20c60..ac69cff 100644
--- a/lib/Providers/Noaa.php
+++ b/lib/Providers/Noaa.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 namespace Ballen\Metar\Providers;
 
 /**
@@ -25,12 +28,13 @@ class Noaa extends MetarHTTPClient implements MetarProviderInterface
     private $serviceUrl = 'http://tgftp.nws.noaa.gov/data/observations/metar/stations/{{_ICAO_}}.TXT';
     private $icao;
 
-    public function __construct($icao)
+    public function __construct(string $icao)
     {
         $this->icao = $icao;
     }
 
-    private function getMetarDataString()
+    /** @throws \GuzzleHttp\Exception\GuzzleException */
+    private function getMetarDataString(): string
     {
 
         $data = $this->getMetarAPIResponse(str_replace('{{_ICAO_}}', $this->icao, $this->serviceUrl));
diff --git a/lib/Providers/Vatsim.php b/lib/Providers/Vatsim.php
index 1ab2778..c229d84 100644
--- a/lib/Providers/Vatsim.php
+++ b/lib/Providers/Vatsim.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 namespace Ballen\Metar\Providers;
 
 /**
@@ -25,12 +28,13 @@ class Vatsim extends MetarHTTPClient implements MetarProviderInterface
     private $serviceUrl = 'http://metar.vatsim.net/metar.php?id={{_ICAO_}}';
     private $icao;
 
-    public function __construct($icao)
+    public function __construct(string $icao)
     {
         $this->icao = $icao;
     }
 
-    private function getMetarDataString()
+    /** @throws \GuzzleHttp\Exception\GuzzleException */
+    private function getMetarDataString(): string
     {
         $data = $this->getMetarAPIResponse(str_replace('{{_ICAO_}}', $this->icao, $this->serviceUrl));
         return trim($data);
diff --git a/tests/MetarHttpClientTest.php b/tests/MetarHttpClientTest.php
index 3e43a51..49cbe82 100644
--- a/tests/MetarHttpClientTest.php
+++ b/tests/MetarHttpClientTest.php
@@ -1,5 +1,8 @@
 <?php
-use GuzzleHttp\Handler\MockHandler;
+
+declare(strict_types=1);
+
+use G0uzzleHttp\Handler\MockHandler;
 use GuzzleHttp\HandlerStack;
 use GuzzleHttp\Psr7\Response;
 use PHPUnit\Framework\TestCase;
@@ -38,7 +41,7 @@ class MetarHttpClientTest extends TestCase
     /**
      * Test recieving a METAR report.
      */
-    public function testDownloadReport()
+    public function testDownloadReport(): void
     {
         $mock = new MockHandler([
             new Response(200, [], self::EXAMPLE_METAR_REPORT),
@@ -51,7 +54,7 @@ public function testDownloadReport()
     /**
      * Test recieving an invalid report.
      */
-    public function testInvalidReport()
+    public function testInvalidReport(): void
     {
         $mock = new MockHandler([
             new Response(404),
@@ -65,7 +68,7 @@ public function testInvalidReport()
     /**
      * Test recieving an unprocessable response (eg. a 502 response).
      */
-    public function testServiceUnavailableRepsonse()
+    public function testServiceUnavailableRepsonse(): void
     {
         $mock = new MockHandler([
             new Response(502),
diff --git a/tests/MetarTest.php b/tests/MetarTest.php
index 2e7c6c1..26ba514 100644
--- a/tests/MetarTest.php
+++ b/tests/MetarTest.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 use Ballen\Metar\Metar;
 use PHPUnit\Framework\TestCase;