diff --git a/src/MutableTypedMap.php b/src/MutableTypedMap.php
new file mode 100644
index 0000000..391910e
--- /dev/null
+++ b/src/MutableTypedMap.php
@@ -0,0 +1,144 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\DataStructures;
+
+/**
+ * @api
+ */
+final class MutableTypedMap extends TypedMap
+{
+    /**
+     * @param array<non-negative-int, mixed> $values
+     */
+    private function __construct(
+        private array $values = [],
+    ) {}
+
+    public static function create(TypedKVPair ...$kvPairs): self
+    {
+        $map = new self();
+
+        foreach ($kvPairs as $pair) {
+            $map->values[$pair->key->index] = $pair->value;
+        }
+
+        return $map;
+    }
+
+    public function with(TypedKey $key, mixed $value): static
+    {
+        $values = $this->values;
+        $values[$key->index] = $value;
+
+        return new self($values);
+    }
+
+    public function withAll(self $map): static
+    {
+        return new self(array_replace($this->values, $map->values));
+    }
+
+    public function without(TypedKey ...$keys): static
+    {
+        $values = $this->values;
+
+        foreach ($keys as $key) {
+            unset($values[$key->index]);
+        }
+
+        return new self($values);
+    }
+
+    public function contains(TypedKey $key): bool
+    {
+        return \array_key_exists($key->index, $this->values);
+    }
+
+    public function offsetExists(mixed $offset): bool
+    {
+        return \array_key_exists($offset->index, $this->values);
+    }
+
+    /**
+     * @template V
+     * @param TypedKey<V> $key
+     * @return V
+     */
+    public function get(TypedKey $key): mixed
+    {
+        if (\array_key_exists($key->index, $this->values)) {
+            /** @var V */
+            return $this->values[$key->index];
+        }
+
+        return $key->default($this);
+    }
+
+    /**
+     * @template V
+     * @param TypedKey<V> $offset
+     * @return V
+     */
+    public function offsetGet(mixed $offset): mixed
+    {
+        if (\array_key_exists($offset->index, $this->values)) {
+            /** @var V */
+            return $this->values[$offset->index];
+        }
+
+        return $offset->default($this);
+    }
+
+    public function count(): int
+    {
+        return \count($this->values);
+    }
+
+    /**
+     * @template V
+     * @param TypedKey<V> $key
+     * @param V $value
+     */
+    public function put(TypedKey $key, mixed $value): void
+    {
+        $this->values[$key->index] = $value;
+    }
+
+    public function putAll(self $map): void
+    {
+        $this->values = array_replace($this->values, $map->values);
+    }
+
+    public function remove(TypedKey ...$keys): void
+    {
+        foreach ($keys as $key) {
+            unset($this->values[$key->index]);
+        }
+    }
+
+    /**
+     * @return list<array{class-string<TypedKey>, non-empty-string, mixed}>
+     */
+    public function __serialize(): array
+    {
+        return array_map(
+            static fn(TypedKey $key, mixed $value): array => [$key::class, $key->method, $value],
+            TypedKey::byIndexes(array_keys($this->values)),
+            $this->values,
+        );
+    }
+
+    /**
+     * @param list<array{class-string<TypedKey>, non-empty-string, mixed}> $data
+     */
+    public function __unserialize(array $data): void
+    {
+        foreach ($data as [$class, $method, $value]) {
+            $key = $class::$method();
+            \assert($key instanceof $class);
+            $this->values[$key->index] = $value;
+        }
+    }
+}
diff --git a/src/TypedKVPair.php b/src/TypedKVPair.php
new file mode 100644
index 0000000..d7e1308
--- /dev/null
+++ b/src/TypedKVPair.php
@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\DataStructures;
+
+/**
+ * @api
+ * @template V
+ */
+final class TypedKVPair
+{
+    /**
+     * @param TypedKey<V> $key
+     * @param V $value
+     */
+    public function __construct(
+        public readonly TypedKey $key,
+        public readonly mixed $value,
+    ) {}
+}
diff --git a/src/TypedKey.php b/src/TypedKey.php
new file mode 100644
index 0000000..baae4a5
--- /dev/null
+++ b/src/TypedKey.php
@@ -0,0 +1,101 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\DataStructures;
+
+/**
+ * @api
+ * @template V
+ * @psalm-consistent-templates
+ */
+abstract class TypedKey
+{
+    /**
+     * @var array<non-empty-string, self>
+     */
+    private static array $keys = [];
+
+    /**
+     * @internal
+     * @psalm-internal Typhoon\DataStructures
+     * @param list<non-negative-int> $indexes
+     * @return list<self>
+     */
+    final public static function byIndexes(array $indexes): array
+    {
+        $keys = array_values(self::$keys);
+
+        return array_map(
+            static fn(int $index): self => $keys[$index] ?? throw new \LogicException(),
+            $indexes,
+        );
+    }
+
+    /**
+     * @template D
+     * @param ?callable(TypedMap): D $default
+     * @return static<D>
+     */
+    final protected static function init(?callable $default = null): static
+    {
+        $trace = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 2)[1] ?? [];
+
+        \assert(
+            isset($trace['class'], $trace['function']) && $trace['class'] === static::class && $trace['function'] !== '',
+            \sprintf('Invalid %s call', self::class),
+        );
+
+        $name = \sprintf('%s::%s', static::class, $trace['function']);
+
+        \assert(!isset(self::$keys[$name]), \sprintf('Please ensure you memoize key in %s()', $name));
+
+        return self::$keys[$name] = new static(
+            index: \count(self::$keys),
+            method: $trace['function'],
+            default: $default ?? static fn(): never => throw new \LogicException(\sprintf('Key %s() does not have a default value', $name)),
+        );
+    }
+
+    /**
+     * @param non-negative-int $index
+     * @param non-empty-string $method
+     * @param callable(TypedMap): V $default
+     */
+    final private function __construct(
+        public readonly int $index,
+        public readonly string $method,
+        private readonly mixed $default,
+    ) {}
+
+    /**
+     * @return V
+     */
+    final public function default(TypedMap $map): mixed
+    {
+        return ($this->default)($map);
+    }
+
+    /**
+     * @return non-empty-string
+     */
+    final public function toString(): string
+    {
+        return self::class . '::' . $this->method . '()';
+    }
+
+    final public function __serialize(): never
+    {
+        throw new \BadMethodCallException(\sprintf('%s does not support serialization', self::class));
+    }
+
+    final public function __unserialize(array $_data): never
+    {
+        throw new \BadMethodCallException(\sprintf('%s does not support deserialization', self::class));
+    }
+
+    final public function __clone()
+    {
+        throw new \BadMethodCallException(\sprintf('%s does not support cloning', self::class));
+    }
+}
diff --git a/src/TypedMap.php b/src/TypedMap.php
new file mode 100644
index 0000000..1419a93
--- /dev/null
+++ b/src/TypedMap.php
@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\DataStructures;
+
+/**
+ * @api
+ * @implements \ArrayAccess<TypedKey, mixed>
+ * @psalm-suppress UnusedClass
+ */
+abstract class TypedMap implements \ArrayAccess, \Countable
+{
+    public static function create(TypedKVPair ...$kvPairs): self
+    {
+        return MutableTypedMap::create(...$kvPairs);
+    }
+
+    /**
+     * @template V
+     * @param TypedKey<V> $key
+     * @param V $value
+     */
+    abstract public function with(TypedKey $key, mixed $value): static;
+
+    abstract public function withAll(MutableTypedMap $map): static;
+
+    abstract public function without(TypedKey ...$keys): static;
+
+    abstract public function contains(TypedKey $key): bool;
+
+    abstract public function offsetExists(mixed $offset): bool;
+
+    /**
+     * @template V
+     * @param TypedKey<V> $key
+     * @return V
+     */
+    abstract public function get(TypedKey $key): mixed;
+
+    /**
+     * @template V
+     * @param TypedKey<V> $offset
+     * @return V
+     */
+    abstract public function offsetGet(mixed $offset): mixed;
+
+    /**
+     * @return non-negative-int
+     */
+    abstract public function count(): int;
+
+    public function offsetSet(mixed $offset, mixed $value): never
+    {
+        throw new \BadMethodCallException();
+    }
+
+    public function offsetUnset(mixed $offset): never
+    {
+        throw new \BadMethodCallException();
+    }
+}