diff --git a/aoc21 b/aoc21
index 9b7903d..ed4d172 100755
--- a/aoc21
+++ b/aoc21
@@ -6,6 +6,7 @@ use AdventOfCode21\Day1;
 use AdventOfCode21\Day2;
 use AdventOfCode21\Day3;
 use AdventOfCode21\Day4;
+use AdventOfCode21\Day6;
 use AdventOfCode21\Puzzle;
 use Symfony\Component\Console\Application;
 
@@ -16,5 +17,6 @@ $application->add(new Day1());
 $application->add(new Day2());
 $application->add(new Day3());
 $application->add(new Day4());
+$application->add(new Day6());
 
 $application->run();
diff --git a/data/day6/data.txt b/data/day6/data.txt
new file mode 100644
index 0000000..ba3c3cc
--- /dev/null
+++ b/data/day6/data.txt
@@ -0,0 +1 @@
+5,4,3,5,1,1,2,1,2,1,3,2,3,4,5,1,2,4,3,2,5,1,4,2,1,1,2,5,4,4,4,1,5,4,5,2,1,2,5,5,4,1,3,1,4,2,4,2,5,1,3,5,3,2,3,1,1,4,5,2,4,3,1,5,5,1,3,1,3,2,2,4,1,3,4,3,3,4,1,3,4,3,4,5,2,1,1,1,4,5,5,1,1,3,2,4,1,2,2,2,4,1,2,5,5,1,4,5,2,4,2,1,5,4,1,3,4,1,2,3,1,5,1,3,4,5,4,1,4,3,3,3,5,5,1,1,5,1,5,5,1,5,2,1,5,1,2,3,5,5,1,3,3,1,5,3,4,3,4,3,2,5,2,1,2,5,1,1,1,1,5,1,1,4,3,3,5,1,1,1,4,4,1,3,3,5,5,4,3,2,1,2,2,3,4,1,5,4,3,1,1,5,1,4,2,3,2,2,3,4,1,3,4,1,4,3,4,3,1,3,3,1,1,4,1,1,1,4,5,3,1,1,2,5,2,5,1,5,3,3,1,3,5,5,1,5,4,3,1,5,1,1,5,5,1,1,2,5,5,5,1,1,3,2,2,3,4,5,5,2,5,4,2,1,5,1,4,4,5,4,4,1,2,1,1,2,3,5,5,1,3,1,4,2,3,3,1,4,1,1
diff --git a/data/day6/example.txt b/data/day6/example.txt
new file mode 100644
index 0000000..55129f1
--- /dev/null
+++ b/data/day6/example.txt
@@ -0,0 +1 @@
+3,4,3,1,2
diff --git a/data/day6/puzzle.md b/data/day6/puzzle.md
new file mode 100644
index 0000000..03f2fa5
--- /dev/null
+++ b/data/day6/puzzle.md
@@ -0,0 +1,68 @@
+# Day 6: Lanternfish
+
+[https://adventofcode.com/2021/day/6](https://adventofcode.com/2021/day/6)
+
+## Description
+
+### Part One
+
+The sea floor is getting steeper. Maybe the sleigh keys got carried this way?
+
+A massive school of glowing [lanternfish](https://en.wikipedia.org/wiki/Lanternfish) swims past. They must spawn quickly to reach such large numbers - maybe _exponentially_ quickly? You should model their growth rate to be sure.
+
+Although you know nothing about this specific species of lanternfish, you make some guesses about their attributes. Surely, <span title="I heard you like lanternfish.">each lanternfish creates a new lanternfish</span> once every _7_ days.
+
+However, this process isn't necessarily synchronized between every lanternfish - one lanternfish might have 2 days left until it creates another lanternfish, while another might have 4. So, you can model each fish as a single number that represents _the number of days until it creates a new lanternfish_.
+
+Furthermore, you reason, a _new_ lanternfish would surely need slightly longer before it's capable of producing more lanternfish: two more days for its first cycle.
+
+So, suppose you have a lanternfish with an internal timer value of `3`:
+
+*   After one day, its internal timer would become `2`.
+*   After another day, its internal timer would become `1`.
+*   After another day, its internal timer would become `0`.
+*   After another day, its internal timer would reset to `6`, and it would create a _new_ lanternfish with an internal timer of `8`.
+*   After another day, the first lanternfish would have an internal timer of `5`, and the second lanternfish would have an internal timer of `7`.
+
+A lanternfish that creates a new fish resets its timer to `6`, _not `7`_ (because `0` is included as a valid timer value). The new lanternfish starts with an internal timer of `8` and does not start counting down until the next day.
+
+Realizing what you're trying to do, the submarine automatically produces a list of the ages of several hundred nearby lanternfish (your puzzle input). For example, suppose you were given the following list:
+
+    3,4,3,1,2
+
+This list means that the first fish has an internal timer of `3`, the second fish has an internal timer of `4`, and so on until the fifth fish, which has an internal timer of `2`. Simulating these fish over several days would proceed as follows:
+
+    Initial state: 3,4,3,1,2
+    After  1 day:  2,3,2,0,1
+    After  2 days: 1,2,1,6,0,8
+    After  3 days: 0,1,0,5,6,7,8
+    After  4 days: 6,0,6,4,5,6,7,8,8
+    After  5 days: 5,6,5,3,4,5,6,7,7,8
+    After  6 days: 4,5,4,2,3,4,5,6,6,7
+    After  7 days: 3,4,3,1,2,3,4,5,5,6
+    After  8 days: 2,3,2,0,1,2,3,4,4,5
+    After  9 days: 1,2,1,6,0,1,2,3,3,4,8
+    After 10 days: 0,1,0,5,6,0,1,2,2,3,7,8
+    After 11 days: 6,0,6,4,5,6,0,1,1,2,6,7,8,8,8
+    After 12 days: 5,6,5,3,4,5,6,0,0,1,5,6,7,7,7,8,8
+    After 13 days: 4,5,4,2,3,4,5,6,6,0,4,5,6,6,6,7,7,8,8
+    After 14 days: 3,4,3,1,2,3,4,5,5,6,3,4,5,5,5,6,6,7,7,8
+    After 15 days: 2,3,2,0,1,2,3,4,4,5,2,3,4,4,4,5,5,6,6,7
+    After 16 days: 1,2,1,6,0,1,2,3,3,4,1,2,3,3,3,4,4,5,5,6,8
+    After 17 days: 0,1,0,5,6,0,1,2,2,3,0,1,2,2,2,3,3,4,4,5,7,8
+    After 18 days: 6,0,6,4,5,6,0,1,1,2,6,0,1,1,1,2,2,3,3,4,6,7,8,8,8,8
+    
+
+Each day, a `0` becomes a `6` and adds a new `8` to the end of the list, while each other number decreases by 1 if it was present at the start of the day.
+
+In this example, after 18 days, there are a total of `26` fish. After 80 days, there would be a total of _`5934`_.
+
+Find a way to simulate lanternfish. _How many lanternfish would there be after 80 days?_
+
+### Part Two
+
+Suppose the lanternfish live forever and have unlimited food and space. Would they take over the entire ocean?
+
+After 256 days in the example above, there would be a total of _`26984457539`_ lanternfish!
+
+_How many lanternfish would there be after 256 days?_
diff --git a/src/Day6.php b/src/Day6.php
new file mode 100644
index 0000000..990a061
--- /dev/null
+++ b/src/Day6.php
@@ -0,0 +1,90 @@
+<?php
+
+namespace AdventOfCode21;
+
+use JetBrains\PhpStorm\Immutable;
+
+class Day6 extends AbstractCommand
+{
+    /**
+     * {@inheritdoc}
+     */
+    protected static int $day = 6;
+
+    /**
+     * @var int[]
+     * @psalm-param array{int: int}
+     */
+    #[Immutable]
+    protected array $startState = [
+        8 => 0,
+        7 => 0,
+        6 => 0,
+        5 => 0,
+        4 => 0,
+        3 => 0,
+        2 => 0,
+        1 => 0,
+        0 => 0,
+    ];
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function part1(array $data): int
+    {
+        return $this->processPuzzle(80, $data[0]);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function part2(array $data): int
+    {
+        return $this->processPuzzle(256, $data[0]);
+    }
+
+    /**
+     * @param int[] $state
+     * @psalm-param array{int: int} $state
+     *
+     * @return array
+     */
+    protected function processDay(array $state): array
+    {
+        $newState = $state;
+
+        /**
+         * @var int $key
+         * @var int $stateValue
+         */
+        foreach ($state as $key => $stateValue) {
+            $newKey = $key - 1;
+
+            if ($newKey < 0) {
+                $newState[8] = $stateValue;
+                $newKey = 6;
+            }
+
+            $newState[$key] -= $stateValue;
+            $newState[$newKey] += $stateValue;
+        }
+
+        return $newState;
+    }
+
+    protected function processPuzzle(int $numberOfDays, string $data): int
+    {
+        $state = $this->startState;
+
+        array_map(static function (string $stateValue) use (&$state) {
+            ++$state[(int) $stateValue];
+        }, explode(',', $data));
+
+        for ($day = 0; $day < $numberOfDays; ++$day) {
+            $state = $this->processDay($state);
+        }
+
+        return (int) array_sum($state);
+    }
+}
diff --git a/tests/Day6Test.php b/tests/Day6Test.php
new file mode 100644
index 0000000..5ee96fa
--- /dev/null
+++ b/tests/Day6Test.php
@@ -0,0 +1,24 @@
+<?php
+
+namespace Tests;
+
+use AdventOfCode21\Day6;
+
+/**
+ * @internal
+ */
+class Day6Test extends AbstractTestCase
+{
+    public static int $part1ExampleResult = 5934;
+    public static int $part1Result = 350917;
+
+    public static int $part2ExampleResult = 26984457539;
+    public static int $part2Result = 1592918715629;
+
+    public function setupDay(): Day6
+    {
+        return new class() extends Day6 {
+            use ReturnTestableResults;
+        };
+    }
+}