diff --git a/src/compound/tagged_union.ts b/src/compound/tagged_union.ts
index d1f21f7..fa247b2 100644
--- a/src/compound/tagged_union.ts
+++ b/src/compound/tagged_union.ts
@@ -11,6 +11,7 @@ type FindDiscriminant<V, D extends number | string> = (variant: V) => D;
 
 type Keys<T> = Exclude<keyof T, symbol>;
 
+/** Union for when the inner type's don't write their own discriminant */
 export class TaggedUnion<
   T extends Record<string | number, UnsizedType<unknown>>,
   V extends ValueOf<{ [K in keyof T]: InnerType<T[K]> }> = ValueOf<
diff --git a/src/compound/tagged_union_test.ts b/src/compound/tagged_union_test.ts
index 4a64201..af9b8d3 100644
--- a/src/compound/tagged_union_test.ts
+++ b/src/compound/tagged_union_test.ts
@@ -41,10 +41,10 @@ Deno.test({
     dt.setBigUint64(0, 0n);
 
     await t.step("Write Packed", () => {
-      type.write(32, dt);
+      type.writePacked(32, dt);
       assertEquals(
         new Uint8Array(ab).subarray(0, 5),
-        Uint8Array.of(0, 0, 0, 0, 32),
+        Uint8Array.of(0, 32, 0, 0, 0),
       );
     });
 
diff --git a/src/compound/union.ts b/src/compound/union.ts
new file mode 100644
index 0000000..ca4dd14
--- /dev/null
+++ b/src/compound/union.ts
@@ -0,0 +1,67 @@
+import { u8 } from "../primitives/mod.ts";
+import {
+  type InnerType,
+  type Options,
+  UnsizedType,
+  type ValueOf,
+} from "../types/mod.ts";
+import { getBiggestAlignment } from "../util.ts";
+
+type FindDiscriminant<V, D extends number> = (variant: V) => D;
+
+type Keys<T> = Exclude<keyof T, symbol | string>;
+
+/** Union for when the inner type's do write their own discriminant */
+export class Union<
+  T extends Record<number, UnsizedType<unknown>>,
+  V extends ValueOf<{ [K in keyof T]: InnerType<T[K]> }>,
+> extends UnsizedType<V> {
+  #record: T;
+  #variantFinder: FindDiscriminant<V, Keys<T>>;
+  #discriminant = u8;
+
+  constructor(
+    input: T,
+    variantFinder: FindDiscriminant<V, Keys<T>>,
+  ) {
+    super(getBiggestAlignment(input));
+    this.#record = input;
+    this.#variantFinder = variantFinder;
+  }
+
+  readPacked(dt: DataView, options: Options = { byteOffset: 0 }): V {
+    const discriminant = this.#discriminant.readPacked(dt, {
+      byteOffset: options.byteOffset,
+    });
+    const codec = this.#record[discriminant];
+    if (!codec) throw new TypeError("Unknown discriminant");
+    return codec.readPacked(dt, options) as V;
+  }
+
+  read(dt: DataView, options: Options = { byteOffset: 0 }): V {
+    const discriminant = this.#discriminant.read(dt, {
+      byteOffset: options.byteOffset,
+    });
+    const codec = this.#record[discriminant];
+    if (!codec) throw new TypeError("Unknown discriminant");
+    return codec.readPacked(dt, options) as V;
+  }
+
+  writePacked(
+    variant: V,
+    dt: DataView,
+    options: Options = { byteOffset: 0 },
+  ): void {
+    const discriminant = this.#variantFinder(variant);
+    const codec = this.#record[discriminant];
+    if (!codec) throw new TypeError("Unknown discriminant");
+    codec.writePacked(variant, dt, options);
+  }
+
+  write(variant: V, dt: DataView, options: Options = { byteOffset: 0 }): void {
+    const discriminant = this.#variantFinder(variant);
+    const codec = this.#record[discriminant];
+    if (!codec) throw new TypeError("Unknown discriminant");
+    codec.write(variant, dt, options);
+  }
+}
diff --git a/src/compound/union_test.ts b/src/compound/union_test.ts
new file mode 100644
index 0000000..3505866
--- /dev/null
+++ b/src/compound/union_test.ts
@@ -0,0 +1,57 @@
+import { u32le, u8 } from "../mod.ts";
+import { assertEquals, assertThrows } from "../../test_deps.ts";
+import { Union } from "./union.ts";
+
+Deno.test({
+  name: "Union",
+  fn: async (t) => {
+    const ab = new ArrayBuffer(8);
+    const dt = new DataView(ab);
+    const type = new Union({
+      0: u32le,
+      1: u8,
+      2: u8,
+    }, (a) => a === 32 ? 0 : 1);
+
+    await t.step("Read", () => {
+      dt.setUint8(0, 1);
+      dt.setUint8(1, 11);
+      dt.setUint8(2, 22);
+      dt.setUint8(4, 33);
+      const result = type.read(dt);
+      assertEquals(result, 1);
+    });
+
+    await t.step("Read Packed", () => {
+      dt.setUint8(0, 1);
+      dt.setUint8(1, 11);
+      dt.setUint8(2, 22);
+      dt.setUint8(4, 33);
+      const result = type.readPacked(dt);
+      assertEquals(result, 1);
+    });
+
+    dt.setBigUint64(0, 0n);
+
+    await t.step("Write", () => {
+      type.write(32, dt);
+      assertEquals(new Uint32Array(ab), Uint32Array.of(32, 0));
+    });
+
+    dt.setBigUint64(0, 0n);
+
+    await t.step("Write Packed", () => {
+      type.writePacked(32, dt);
+      assertEquals(
+        new Uint8Array(ab).subarray(0, 5),
+        Uint8Array.of(32, 0, 0, 0, 0),
+      );
+    });
+
+    await t.step("OOB Read", () => {
+      assertThrows(() => {
+        type.read(dt, { byteOffset: 9 });
+      }, RangeError);
+    });
+  },
+});