diff --git a/README.md b/README.md
index eb9e32b..22e621e 100644
--- a/README.md
+++ b/README.md
@@ -124,6 +124,8 @@ All entry points always return an instance of the pipeline.
 | `flip()`    | Swaps keys and values. | `array_flip` |
 | `max()`     | Finds the highest value. | `max` |
 | `min()`     | Finds the lowest value. | `min` |
+| `count()`     | Counts value. | `array_count` |
+| `runningCount()` | Counts seen values using a reference argument. | |
 | `toArray()` | Returns an array with all values. Eagerly executed. | `dict`, `ToDictionary` |
 | `toArrayPreservingKeys()` | Returns an array with all values and keys. Eagerly executed. |  |
 | `runningVariance()` | Computes online statistics: sample mean, sample variance, standard deviation. | [Welford's method](https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm) |
diff --git a/src/Standard.php b/src/Standard.php
index d9fda55..ccc3e36 100644
--- a/src/Standard.php
+++ b/src/Standard.php
@@ -547,6 +547,29 @@ public function toArrayPreservingKeys(): array
         return $this->toArray(true);
     }
 
+    /**
+     * Counts seen values online.
+     *
+     * @param ?int &$count the current count; initialized unless provided
+     *
+     * @param-out int $count
+     *
+     * @return $this
+     */
+    public function runningCount(
+        ?int &$count
+    ): self {
+        $count ??= 0;
+
+        $this->cast(static function ($input) use (&$count) {
+            ++$count;
+
+            return $input;
+        });
+
+        return $this;
+    }
+
     /**
      * {@inheritdoc}
      *
diff --git a/tests/CountTest.php b/tests/CountTest.php
index 62d2aeb..8d31317 100644
--- a/tests/CountTest.php
+++ b/tests/CountTest.php
@@ -25,6 +25,7 @@
 use function Pipeline\fromArray;
 use function Pipeline\map;
 use function Pipeline\take;
+use function range;
 
 /**
  * @covers \Pipeline\Standard
@@ -61,4 +62,23 @@ public function testCountValues(): void
             yield 2 => 3;
         }));
     }
+
+    public function testRunningCount(): void
+    {
+        $countEven = 1;
+
+        $pipeline = map(fn () => yield from range(0, 100))
+            ->runningCount($countAll)
+            ->filter(fn (int $n) => $n % 2)
+            ->runningCount($countEven)
+            ->filter(fn (int $n) => $n % 3);
+
+        $this->assertSame(0, $countAll);
+        $this->assertSame(1, $countEven);
+
+        $this->assertSame(33, $pipeline->count());
+
+        $this->assertSame(101, $countAll);
+        $this->assertSame(50, $countEven - 1);
+    }
 }