diff --git a/src/Coroutine.php b/src/Coroutine.php
index 0da0228..0f49829 100644
--- a/src/Coroutine.php
+++ b/src/Coroutine.php
@@ -42,6 +42,8 @@
  * @return Promise
  *
  * @see https://github.com/petkaantonov/bluebird/blob/master/API.md#generators inspiration
+ * @template T
+ * @template-implements PromiseInterface<T>
  */
 final class Coroutine implements PromiseInterface
 {
diff --git a/src/FulfilledPromise.php b/src/FulfilledPromise.php
index 727ec31..3197b23 100644
--- a/src/FulfilledPromise.php
+++ b/src/FulfilledPromise.php
@@ -10,6 +10,9 @@
  * Thenning off of this promise will invoke the onFulfilled callback
  * immediately and ignore other callbacks.
  *
+ * @template T
+ * @template-implements PromiseInterface<T>
+ *
  * @final
  */
 class FulfilledPromise implements PromiseInterface
@@ -17,7 +20,7 @@ class FulfilledPromise implements PromiseInterface
     private $value;
 
     /**
-     * @param mixed $value
+     * @param T $value
      */
     public function __construct($value)
     {
diff --git a/src/Promise.php b/src/Promise.php
index c0c5be2..d0f299d 100644
--- a/src/Promise.php
+++ b/src/Promise.php
@@ -8,6 +8,8 @@
  * Promises/A+ implementation that avoids recursion when possible.
  *
  * @see https://promisesaplus.com/
+ * @template T
+ * @template-implements PromiseInterface<T>
  *
  * @final
  */
diff --git a/src/PromiseInterface.php b/src/PromiseInterface.php
index c11721e..ed62bb3 100644
--- a/src/PromiseInterface.php
+++ b/src/PromiseInterface.php
@@ -12,6 +12,7 @@
  * the reason why the promise cannot be fulfilled.
  *
  * @see https://promisesaplus.com/
+ * @template T
  */
 interface PromiseInterface
 {
@@ -23,8 +24,12 @@ interface PromiseInterface
      * Appends fulfillment and rejection handlers to the promise, and returns
      * a new promise resolving to the return value of the called handler.
      *
-     * @param callable $onFulfilled Invoked when the promise fulfills.
-     * @param callable $onRejected  Invoked when the promise is rejected.
+     * @template U
+     * @template V
+     * @param null|callable(T): U $onFulfilled Invoked when the promise fulfills.
+     * @param null|callable(mixed): V $onRejected  Invoked when the promise is rejected.
+     *
+     * @return ($onFulfilled is null ? ($onRejected is null ? PromiseInterface<T> : PromiseInterface<T|V>) : ($onRejected is null ? PromiseInterface<T|U> : PromiseInterface<U|V>))
      */
     public function then(
         ?callable $onFulfilled = null,
@@ -37,7 +42,10 @@ public function then(
      * or to its original fulfillment value if the promise is instead
      * fulfilled.
      *
-     * @param callable $onRejected Invoked when the promise is rejected.
+     * @template V
+     * @param callable(mixed): V $onRejected Invoked when the promise is rejected.
+     *
+     * @return PromiseInterface<T|V>
      */
     public function otherwise(callable $onRejected): PromiseInterface;
 
@@ -46,13 +54,15 @@ public function otherwise(callable $onRejected): PromiseInterface;
      *
      * The three states can be checked against the constants defined on
      * PromiseInterface: PENDING, FULFILLED, and REJECTED.
+     *
+     * @return 'pending'|'fulfilled'|'rejected'
      */
     public function getState(): string;
 
     /**
      * Resolve the promise with the given value.
      *
-     * @param mixed $value
+     * @param T $value
      *
      * @throws \RuntimeException if the promise is already resolved.
      */
@@ -82,7 +92,9 @@ public function cancel(): void;
      *
      * If the promise cannot be waited on, then the promise will be rejected.
      *
-     * @return mixed
+     * @param bool $unwrap
+     *
+     * @return ($unwrap is true ? T : void)
      *
      * @throws \LogicException if the promise has no wait function or if the
      *                         promise does not settle after waiting.
diff --git a/src/RejectedPromise.php b/src/RejectedPromise.php
index 1ebf0b2..36853cc 100644
--- a/src/RejectedPromise.php
+++ b/src/RejectedPromise.php
@@ -10,6 +10,8 @@
  * Thenning off of this promise will invoke the onRejected callback
  * immediately and ignore other callbacks.
  *
+ * @template-implements PromiseInterface<null>
+ *
  * @final
  */
 class RejectedPromise implements PromiseInterface