diff --git a/src/Negotiation/AbstractNegotiator.php b/src/Negotiation/AbstractNegotiator.php
index 669941e..6059ecd 100644
--- a/src/Negotiation/AbstractNegotiator.php
+++ b/src/Negotiation/AbstractNegotiator.php
@@ -13,13 +13,13 @@ abstract class AbstractNegotiator
      *
      * @return AcceptHeader|null best matching type
      */
-    public function getBest($header, array $priorities, $strict = false)
+    public function getBest(string $header, array $priorities,bool $strict = false): ?AcceptHeader
     {
-        if (empty($priorities)) {
+        if ($priorities === []) {
             throw new InvalidArgument('A set of server priorities should be given.');
         }
 
-        if (!$header) {
+        if ($header === '') {
             throw new InvalidArgument('The header string should not be empty.');
         }
 
@@ -55,9 +55,9 @@ public function getBest($header, array $priorities, $strict = false)
     /**
      * @param string $header A string containing an `Accept|Accept-*` header.
      *
-     * @return [AcceptHeader] An ordered list of accept header elements
+     * @return AcceptHeader[] An ordered list of accept header elements
      */
-    public function getOrderedElements($header)
+    public function getOrderedElements(string $header): array
     {
         if (!$header) {
             throw new InvalidArgument('The header string should not be empty.');
@@ -111,7 +111,7 @@ abstract protected function acceptFactory($header);
      *
      * @return Match|null Headers matched
      */
-    protected function match(AcceptHeader $header, AcceptHeader $priority, $index)
+    protected function match(AcceptHeader $header, AcceptHeader $priority, $index): ?Match
     {
         $ac = $header->getType();
         $pc = $priority->getType();
@@ -132,7 +132,7 @@ protected function match(AcceptHeader $header, AcceptHeader $priority, $index)
      *
      * @return AcceptHeader[]
      */
-    private function parseHeader($header)
+    private function parseHeader($header): array
     {
         $res = preg_match_all('/(?:[^,"]*+(?:"[^"]*+")?)+[^,"]*+/', $header, $matches);
 
@@ -149,7 +149,7 @@ private function parseHeader($header)
      *
      * @return Match[] Headers matched
      */
-    private function findMatches(array $headerParts, array $priorities)
+    private function findMatches(array $headerParts, array $priorities): array
     {
         $matches = [];
         foreach ($priorities as $index => $p) {
diff --git a/src/Negotiation/Accept.php b/src/Negotiation/Accept.php
index 281ae27..9624fa4 100644
--- a/src/Negotiation/Accept.php
+++ b/src/Negotiation/Accept.php
@@ -31,7 +31,7 @@ public function __construct($value)
     /**
      * @return string
      */
-    public function getSubPart()
+    public function getSubPart(): string
     {
         return $this->subPart;
     }
@@ -39,7 +39,7 @@ public function getSubPart()
     /**
      * @return string
      */
-    public function getBasePart()
+    public function getBasePart(): string
     {
         return $this->basePart;
     }
diff --git a/src/Negotiation/BaseAccept.php b/src/Negotiation/BaseAccept.php
index 3c58447..08c45b2 100644
--- a/src/Negotiation/BaseAccept.php
+++ b/src/Negotiation/BaseAccept.php
@@ -32,7 +32,7 @@ abstract class BaseAccept
     /**
      * @param string $value
      */
-    public function __construct($value)
+    public function __construct(string $value)
     {
         list($type, $parameters) = $this->parseParameters($value);
 
@@ -52,7 +52,7 @@ public function __construct($value)
     /**
      * @return string
      */
-    public function getNormalizedValue()
+    public function getNormalizedValue(): string
     {
         return $this->normalized;
     }
@@ -60,7 +60,7 @@ public function getNormalizedValue()
     /**
      * @return string
      */
-    public function getValue()
+    public function getValue(): string
     {
         return $this->value;
     }
@@ -68,7 +68,7 @@ public function getValue()
     /**
      * @return string
      */
-    public function getType()
+    public function getType(): string
     {
         return $this->type;
     }
@@ -76,7 +76,7 @@ public function getType()
     /**
      * @return float
      */
-    public function getQuality()
+    public function getQuality(): float
     {
         return $this->quality;
     }
@@ -84,7 +84,7 @@ public function getQuality()
     /**
      * @return array
      */
-    public function getParameters()
+    public function getParameters(): array
     {
         return $this->parameters;
     }
@@ -95,7 +95,7 @@ public function getParameters()
      *
      * @return string|null
      */
-    public function getParameter($key, $default = null)
+    public function getParameter(string $key, $default = null): ?string
     {
         return isset($this->parameters[$key]) ? $this->parameters[$key] : $default;
     }
@@ -105,7 +105,7 @@ public function getParameter($key, $default = null)
      *
      * @return boolean
      */
-    public function hasParameter($key)
+    public function hasParameter(string $key): bool
     {
         return isset($this->parameters[$key]);
     }
@@ -115,7 +115,7 @@ public function hasParameter($key)
      * @param  string $acceptPart
      * @return array
      */
-    private function parseParameters($acceptPart)
+    private function parseParameters(string $acceptPart): array
     {
         $parts = explode(';', $acceptPart);
         $type  = array_shift($parts);
@@ -140,7 +140,7 @@ private function parseParameters($acceptPart)
      *
      * @return string
      */
-    private function buildParametersString($parameters)
+    private function buildParametersString($parameters): string
     {
         $parts = [];
 
diff --git a/src/Negotiation/CharsetNegotiator.php b/src/Negotiation/CharsetNegotiator.php
index c1f9a4b..7c110ef 100644
--- a/src/Negotiation/CharsetNegotiator.php
+++ b/src/Negotiation/CharsetNegotiator.php
@@ -7,7 +7,7 @@ class CharsetNegotiator extends AbstractNegotiator
     /**
      * {@inheritdoc}
      */
-    protected function acceptFactory($accept)
+    protected function acceptFactory($accept): AcceptCharset
     {
         return new AcceptCharset($accept);
     }
diff --git a/src/Negotiation/EncodingNegotiator.php b/src/Negotiation/EncodingNegotiator.php
index 6b97fb7..3344dda 100644
--- a/src/Negotiation/EncodingNegotiator.php
+++ b/src/Negotiation/EncodingNegotiator.php
@@ -7,7 +7,7 @@ class EncodingNegotiator extends AbstractNegotiator
     /**
      * {@inheritdoc}
      */
-    protected function acceptFactory($accept)
+    protected function acceptFactory($accept): AcceptEncoding
     {
         return new AcceptEncoding($accept);
     }
diff --git a/src/Negotiation/LanguageNegotiator.php b/src/Negotiation/LanguageNegotiator.php
index ac93814..7ab0ffd 100644
--- a/src/Negotiation/LanguageNegotiator.php
+++ b/src/Negotiation/LanguageNegotiator.php
@@ -7,7 +7,7 @@ class LanguageNegotiator extends AbstractNegotiator
     /**
      * {@inheritdoc}
      */
-    protected function acceptFactory($accept)
+    protected function acceptFactory($accept): AccepLanguage
     {
         return new AcceptLanguage($accept);
     }
@@ -15,7 +15,7 @@ protected function acceptFactory($accept)
     /**
      * {@inheritdoc}
      */
-    protected function match(AcceptHeader $acceptLanguage, AcceptHeader $priority, $index)
+    protected function match(AcceptHeader $acceptLanguage, AcceptHeader $priority, $index): ?Match
     {
         if (!$acceptLanguage instanceof AcceptLanguage || !$priority instanceof AcceptLanguage) {
             return null;
diff --git a/src/Negotiation/Match.php b/src/Negotiation/Match.php
index 5b0e014..0300ec3 100644
--- a/src/Negotiation/Match.php
+++ b/src/Negotiation/Match.php
@@ -51,7 +51,7 @@ public static function compare(Match $a, Match $b)
      *
      * @return Match[]
      */
-    public static function reduce(array $carry, Match $match)
+    public static function reduce(array $carry, Match $match): array
     {
         if (!isset($carry[$match->index]) || $carry[$match->index]->score < $match->score) {
             $carry[$match->index] = $match;
diff --git a/src/Negotiation/Negotiator.php b/src/Negotiation/Negotiator.php
index d083fa1..f6ae1fe 100644
--- a/src/Negotiation/Negotiator.php
+++ b/src/Negotiation/Negotiator.php
@@ -7,7 +7,7 @@ class Negotiator extends AbstractNegotiator
     /**
      * {@inheritdoc}
      */
-    protected function acceptFactory($accept)
+    protected function acceptFactory($accept): Accept
     {
         return new Accept($accept);
     }
@@ -15,7 +15,7 @@ protected function acceptFactory($accept)
     /**
      * {@inheritdoc}
      */
-    protected function match(AcceptHeader $accept, AcceptHeader $priority, $index)
+    protected function match(AcceptHeader $accept, AcceptHeader $priority, $index): ?Match
     {
         if (!$accept instanceof Accept || !$priority instanceof Accept) {
             return null;
@@ -78,7 +78,7 @@ protected function match(AcceptHeader $accept, AcceptHeader $priority, $index)
      * should allow wildcards for either the portion before the "+" or
      * after. This method splits the subpart to allow such matching.
      */
-    protected function splitSubPart($subPart)
+    protected function splitSubPart($subPart): array
     {
         if (!strstr($subPart, '+')) {
             return [$subPart, ''];