diff --git a/src/Internal/ArrayTypedMap.php b/src/Internal/ArrayTypedMap.php
new file mode 100644
index 0000000..cb6505d
--- /dev/null
+++ b/src/Internal/ArrayTypedMap.php
@@ -0,0 +1,88 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\DataStructures\Internal;
+
+use Typhoon\DataStructures\MutableTypedMap;
+use Typhoon\DataStructures\TypedKey;
+use Typhoon\DataStructures\TypedMap;
+
+/**
+ * @internal
+ * @psalm-internal Typhoon\DataStructures
+ */
+final class ArrayTypedMap extends MutableTypedMap
+{
+    /**
+     * @var array<non-empty-string, mixed>
+     */
+    private array $values = [];
+
+    /**
+     * @return non-empty-string
+     */
+    private static function keyToString(TypedKey $key): string
+    {
+        return $key::class . '::' . $key->name;
+    }
+
+    protected function doPut(TypedKey $key, mixed $value): void
+    {
+        $this->values[self::keyToString($key)] = $value;
+    }
+
+    public function putAll(TypedMap $map): void
+    {
+        if ($map instanceof self) {
+            $this->values = [...$this->values, ...$map->values];
+        } else {
+            parent::putAll($map);
+        }
+    }
+
+    public function contains(TypedKey $key): bool
+    {
+        return isset($this->values[self::keyToString($key)]);
+    }
+
+    /**
+     * @template V
+     * @template D
+     * @param TypedKey<V> $key
+     * @param callable(): D $or
+     * @return V|D
+     */
+    public function getOr(TypedKey $key, callable $or): mixed
+    {
+        $keyString = self::keyToString($key);
+
+        if (\array_key_exists($keyString, $this->values)) {
+            /** @var V */
+            return $this->values[$keyString];
+        }
+
+        return $or();
+    }
+
+    public function remove(TypedKey ...$keys): void
+    {
+        foreach ($keys as $key) {
+            unset($this->values[self::keyToString($key)]);
+        }
+    }
+
+    public function count(): int
+    {
+        return \count($this->values);
+    }
+
+    protected function all(): \Traversable
+    {
+        foreach ($this->values as $keyString => $value) {
+            /** @var TypedKey $key */
+            $key = \constant($keyString);
+            yield $key => $value;
+        }
+    }
+}
diff --git a/src/Internal/SplObjectStorageTypedMap.php b/src/Internal/SplObjectStorageTypedMap.php
new file mode 100644
index 0000000..ebc9fcc
--- /dev/null
+++ b/src/Internal/SplObjectStorageTypedMap.php
@@ -0,0 +1,83 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\DataStructures\Internal;
+
+use Typhoon\DataStructures\MutableTypedMap;
+use Typhoon\DataStructures\TypedKey;
+use Typhoon\DataStructures\TypedMap;
+
+/**
+ * @internal
+ * @psalm-internal Typhoon\DataStructures
+ * @psalm-suppress UnusedClass
+ */
+final class SplObjectStorageTypedMap extends MutableTypedMap
+{
+    /**
+     * @var \SplObjectStorage<TypedKey, mixed>
+     */
+    private \SplObjectStorage $values;
+
+    public function __construct()
+    {
+        /** @var \SplObjectStorage<TypedKey, mixed> */
+        $this->values = new \SplObjectStorage();
+    }
+
+    protected function doPut(TypedKey $key, mixed $value): void
+    {
+        $this->values->attach($key, $value);
+    }
+
+    public function contains(TypedKey $key): bool
+    {
+        return $this->values->contains($key);
+    }
+
+    /**
+     * @template V
+     * @template D
+     * @param TypedKey<V> $key
+     * @param callable(): D $or
+     * @return V|D
+     */
+    public function getOr(mixed $key, callable $or): mixed
+    {
+        if ($this->values->contains($key)) {
+            /** @var V */
+            return $this->values->offsetGet($key);
+        }
+
+        return $or();
+    }
+
+    public function remove(TypedKey ...$keys): void
+    {
+        foreach ($keys as $key) {
+            $this->values->detach($key);
+        }
+    }
+
+    public function putAll(TypedMap $map): void
+    {
+        if ($map instanceof self) {
+            $this->values->addAll($map->values);
+        } else {
+            parent::putAll($map);
+        }
+    }
+
+    public function count(): int
+    {
+        return \count($this->values);
+    }
+
+    protected function all(): \Traversable
+    {
+        foreach ($this->values as $key) {
+            yield $key => $this->values->getInfo();
+        }
+    }
+}
diff --git a/src/MutableTypedMap.php b/src/MutableTypedMap.php
new file mode 100644
index 0000000..13b919a
--- /dev/null
+++ b/src/MutableTypedMap.php
@@ -0,0 +1,87 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\DataStructures;
+
+/**
+ * @api
+ */
+abstract class MutableTypedMap extends TypedMap
+{
+    /**
+     * @template V
+     * @param TypedKey<V> $key
+     * @param V $value
+     */
+    final public function with(TypedKey $key, mixed $value): static
+    {
+        if ($key instanceof OptionalTypedKey && $value === $key->default($this)) {
+            if ($this->contains($key)) {
+                $copy = clone $this;
+                $copy->remove($key);
+
+                return $copy;
+            }
+
+            return $this;
+        }
+
+        $copy = clone $this;
+        $copy->doPut($key, $value);
+
+        return $copy;
+    }
+
+    final public function withAll(TypedMap $map): static
+    {
+        $copy = clone $this;
+        foreach ($map->all() as $key => $value) {
+            $copy->put($key, $value);
+        }
+
+        return $copy;
+    }
+
+    final public function without(TypedKey ...$keys): static
+    {
+        if ($keys === []) {
+            return $this;
+        }
+
+        $copy = clone $this;
+        $copy->remove(...$keys);
+
+        return $copy;
+    }
+
+    /**
+     * @template V
+     * @param TypedKey<V> $key
+     * @param V $value
+     */
+    final public function put(TypedKey $key, mixed $value): void
+    {
+        if ($key instanceof OptionalTypedKey && $value === $key->default($this)) {
+            $this->remove($key);
+        } else {
+            $this->doPut($key, $value);
+        }
+    }
+
+    /**
+     * @template V
+     * @param TypedKey<V> $key
+     * @param V $value
+     */
+    abstract protected function doPut(TypedKey $key, mixed $value): void;
+
+    public function putAll(TypedMap $map): void
+    {
+        foreach ($map->all() as $key => $value) {
+            $this->put($key, $value);
+        }
+    }
+
+    abstract public function remove(TypedKey ...$keys): void;
+}
diff --git a/src/OptionalTypedKey.php b/src/OptionalTypedKey.php
new file mode 100644
index 0000000..5eecfb8
--- /dev/null
+++ b/src/OptionalTypedKey.php
@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\DataStructures;
+
+/**
+ * @api
+ * @template-covariant TValue
+ * @extends TypedKey<TValue>
+ */
+interface OptionalTypedKey extends TypedKey
+{
+    /**
+     * @return TValue
+     */
+    public function default(TypedMap $map): mixed;
+}
diff --git a/src/TypedKey.php b/src/TypedKey.php
new file mode 100644
index 0000000..bc91e9b
--- /dev/null
+++ b/src/TypedKey.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\DataStructures;
+
+/**
+ * @api
+ * @template-covariant TValue
+ */
+interface TypedKey extends \UnitEnum {}
diff --git a/src/TypedMap.php b/src/TypedMap.php
new file mode 100644
index 0000000..32e8651
--- /dev/null
+++ b/src/TypedMap.php
@@ -0,0 +1,91 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\DataStructures;
+
+use Typhoon\DataStructures\Internal\ArrayTypedMap;
+
+/**
+ * @api
+ * @implements \ArrayAccess<TypedKey, mixed>
+ * @psalm-suppress UnusedClass
+ */
+abstract class TypedMap implements \ArrayAccess, \Countable
+{
+    public static function create(): static
+    {
+        /** @var static */
+        return new ArrayTypedMap();
+    }
+
+    /**
+     * @template V
+     * @param TypedKey<V> $key
+     * @param V $value
+     */
+    abstract public function with(TypedKey $key, mixed $value): static;
+
+    abstract public function withAll(self $map): static;
+
+    abstract public function without(TypedKey ...$keys): static;
+
+    abstract public function contains(TypedKey $key): bool;
+
+    final public function offsetExists(mixed $offset): bool
+    {
+        return $this->contains($offset);
+    }
+
+    /**
+     * @template V
+     * @template D
+     * @param TypedKey<V> $key
+     * @param D $default
+     * @return V|D
+     */
+    final public function get(TypedKey $key, mixed $default = null): mixed
+    {
+        return $this->getOr($key, static fn(): mixed => $default);
+    }
+
+    /**
+     * @template V
+     * @template D
+     * @param TypedKey<V> $key
+     * @param callable(): D $or
+     * @return V|D
+     */
+    abstract public function getOr(TypedKey $key, callable $or): mixed;
+
+    /**
+     * @template V
+     * @param TypedKey<V> $offset
+     * @return V
+     * @throws KeyIsNotDefined
+     */
+    final public function offsetGet(mixed $offset): mixed
+    {
+        return $this->getOr($offset, static function () use ($offset): void { throw new KeyIsNotDefined($offset); });
+    }
+
+    /**
+     * @return non-negative-int
+     */
+    abstract public function count(): int;
+
+    /**
+     * @return \Traversable<TypedKey, mixed>
+     */
+    abstract protected function all(): \Traversable;
+
+    public function offsetSet(mixed $offset, mixed $value): never
+    {
+        throw new \BadMethodCallException();
+    }
+
+    public function offsetUnset(mixed $offset): never
+    {
+        throw new \BadMethodCallException();
+    }
+}