diff --git a/src/GettextTranslator.php b/src/GettextTranslator.php
index a7a8144..3a8f2ca 100644
--- a/src/GettextTranslator.php
+++ b/src/GettextTranslator.php
@@ -219,7 +219,7 @@ public function encodeMessageWithContext($message, $context)
         return "{$context}\x04{$message}";
     }
 
-    public function translate($message, $context = null)
+    public function translate(string $message, ?string $context = null): string
     {
         if ($context !== null) {
             $messageForGettext = $this->encodeMessageWithContext($message, $context);
@@ -236,7 +236,7 @@ public function translate($message, $context = null)
         return $translation;
     }
 
-    public function translateInDomain($domain, $message, $context = null)
+    public function translateInDomain(string $domain, string $message, ?string $context = null): string
     {
         if ($context !== null) {
             $messageForGettext = $this->encodeMessageWithContext($message, $context);
@@ -263,7 +263,7 @@ public function translateInDomain($domain, $message, $context = null)
         return $translation;
     }
 
-    public function translatePlural($singular, $plural, $number, $context = null)
+    public function translatePlural(string $singular, string $plural, int $number, ?string $context = null): string
     {
         if ($context !== null) {
             $singularForGettext = $this->encodeMessageWithContext($singular, $context);
@@ -285,8 +285,13 @@ public function translatePlural($singular, $plural, $number, $context = null)
         return $translation;
     }
 
-    public function translatePluralInDomain($domain, $singular, $plural, $number, $context = null)
-    {
+    public function translatePluralInDomain(
+        string $domain,
+        string $singular,
+        string $plural,
+        int $number,
+        ?string $context = null
+    ): string {
         if ($context !== null) {
             $singularForGettext = $this->encodeMessageWithContext($singular, $context);
         } else {
diff --git a/src/Locale.php b/src/Locale.php
index d9e6b05..51958b0 100644
--- a/src/Locale.php
+++ b/src/Locale.php
@@ -12,7 +12,7 @@ class Locale
      *
      * @return string
      */
-    public function getDefaultLocale()
+    public function getDefaultLocale(): string
     {
         return $this->defaultLocale;
     }
@@ -24,7 +24,7 @@ public function getDefaultLocale()
      *
      * @return $this
      */
-    public function setDefaultLocale($defaultLocale)
+    public function setDefaultLocale(string $defaultLocale): self
     {
         $this->defaultLocale = $defaultLocale;
 
@@ -39,7 +39,7 @@ public function setDefaultLocale($defaultLocale)
      *
      * @return string The browser's preferred locale code
      */
-    public function getPreferred($header, array $available)
+    public function getPreferred(string $header, array $available): string
     {
         $headerValues = explode(',', $header);
         for ($i = 0; $i < count($headerValues); $i++) {
@@ -117,7 +117,7 @@ function ($a, $b) {
      *
      * @return object Output of {@link \Locale::parseLocale()} converted to an object
      */
-    public function parseLocale($locale)
+    public function parseLocale(string $locale): object
     {
         return (object) \Locale::parseLocale($locale);
     }
diff --git a/src/NoopTranslator.php b/src/NoopTranslator.php
index 1f9aab2..635ccad 100644
--- a/src/NoopTranslator.php
+++ b/src/NoopTranslator.php
@@ -9,23 +9,28 @@
  */
 class NoopTranslator implements Translator
 {
-    public function translate($message, $context = null)
+    public function translate(string $message, ?string $context = null): string
     {
         return $message;
     }
 
-    public function translateInDomain($domain, $message, $context = null)
+    public function translateInDomain(string $domain, string $message, ?string $context = null): string
     {
         return $message;
     }
 
-    public function translatePlural($singular, $plural, $number, $context = null)
+    public function translatePlural(string $singular, string $plural, int $number, ?string $context = null): string
     {
         return $number === 1 ? $singular : $plural;
     }
 
-    public function translatePluralInDomain($domain, $singular, $plural, $number, $context = null)
-    {
+    public function translatePluralInDomain(
+        string $domain,
+        string $singular,
+        string $plural,
+        int $number,
+        ?string $context = null
+    ): string {
         return $number === 1 ? $singular : $plural;
     }
 }
diff --git a/src/Translation.php b/src/Translation.php
index eb40287..8d49ed0 100644
--- a/src/Translation.php
+++ b/src/Translation.php
@@ -18,12 +18,12 @@ trait Translation
     /**
      * Translate a message
      *
-     * @param string $message
-     * @param string $context Message context
+     * @param string  $message
+     * @param ?string $context Message context
      *
      * @return string Translated message or original message if no translation is found
      */
-    public function translate($message, $context = null)
+    public function translate(string $message, ?string $context = null): string
     {
         return $this->translationDomain === null
             ? StaticTranslator::$instance->translate($message, $context)
@@ -35,13 +35,13 @@ public function translate($message, $context = null)
      *
      * If no translation is found in the specified domain, the translation is also searched for in the default domain.
      *
-     * @param string $domain
-     * @param string $message
-     * @param string $context Message context
+     * @param string  $domain
+     * @param string  $message
+     * @param ?string $context Message context
      *
      * @return string Translated message or original message if no translation is found
      */
-    public function translateInDomain($domain, $message, $context = null)
+    public function translateInDomain(string $domain, string $message, ?string $context = null): string
     {
         return StaticTranslator::$instance->translateInDomain($domain, $message, $context);
     }
@@ -52,14 +52,14 @@ public function translateInDomain($domain, $message, $context = null)
      * The returned message is based on the given number to decide between the singular and plural forms.
      * That is also the case if no translation is found.
      *
-     * @param string $singular Singular message
-     * @param string $plural   Plural message
-     * @param ?int   $number   Number to decide between the returned singular and plural forms
-     * @param string $context  Message context
+     * @param string  $singular Singular message
+     * @param string  $plural   Plural message
+     * @param ?int    $number   Number to decide between the returned singular and plural forms
+     * @param ?string $context  Message context
      *
      * @return string Translated message or original message if no translation is found
      */
-    public function translatePlural($singular, $plural, $number, $context = null)
+    public function translatePlural(string $singular, string $plural, ?int $number, ?string $context = null): string
     {
         return $this->translationDomain === null
             ? StaticTranslator::$instance->translatePlural($singular, $plural, $number ?? 0, $context)
@@ -80,21 +80,26 @@ public function translatePlural($singular, $plural, $number, $context = null)
      * The returned message is based on the given number to decide between the singular and plural forms.
      * That is also the case if no translation is found.
      *
-     * @param string $domain
-     * @param string $singular Singular message
-     * @param string $plural   Plural message
-     * @param ?int   $number   Number to decide between the returned singular and plural forms
-     * @param string $context  Message context
+     * @param string  $domain
+     * @param string  $singular Singular message
+     * @param string  $plural   Plural message
+     * @param ?int    $number   Number to decide between the returned singular and plural forms
+     * @param ?string $context  Message context
      *
      * @return string Translated message or original message if no translation is found
      */
-    public function translatePluralInDomain($domain, $singular, $plural, $number, $context = null)
-    {
+    public function translatePluralInDomain(
+        string $domain,
+        string $singular,
+        string $plural,
+        ?int $number,
+        ?string $context = null
+    ): string {
         return StaticTranslator::$instance->translatePluralInDomain(
             $domain,
             $singular,
             $plural,
-            $number ?? 0,
+                $number ?? 0,
             $context
         );
     }
diff --git a/src/functions.php b/src/functions.php
index 74d58df..9730bbf 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -5,12 +5,12 @@
 /**
  * Translate a message
  *
- * @param string $message
- * @param string $context Message context
+ * @param string  $message
+ * @param ?string $context Message context
  *
  * @return string Translated message or original message if no translation is found
  */
-function t($message, $context = null)
+function t(string $message, ?string $context = null): string
 {
     return StaticTranslator::$instance->translate($message, $context);
 }
@@ -21,14 +21,14 @@ function t($message, $context = null)
  * The returned message is based on the given number to decide between the singular and plural forms.
  * That is also the case if no translation is found.
  *
- * @param string $singular Singular message
- * @param string $plural   Plural message
- * @param int    $number   Number to decide between the returned singular and plural forms
- * @param string $context  Message context
+ * @param string  $singular Singular message
+ * @param string  $plural   Plural message
+ * @param ?int    $number   Number to decide between the returned singular and plural forms
+ * @param ?string $context  Message context
  *
  * @return string Translated message or original message if no translation is found
  */
-function tp($singular, $plural, $number, $context = null)
+function tp(string $singular, string $plural, ?int $number, ?string $context = null): string
 {
-    return StaticTranslator::$instance->translatePlural($singular, $plural, $number, $context);
+    return StaticTranslator::$instance->translatePlural($singular, $plural, $number ?? 0, $context);
 }