diff --git a/source/group.ts b/source/group.ts
new file mode 100644
index 0000000..8dabe9c
--- /dev/null
+++ b/source/group.ts
@@ -0,0 +1,47 @@
+import {type Result, Test, TestContext} from "./test.js";
+
+/** Create a new test group and run it. */
+export async function setup(
+  name: string,
+  fn: (group: Group) => Promise<void>,
+): Promise<Group> {
+  const group = new Group(name);
+  await fn(group);
+  await group.run();
+  return group;
+}
+
+/** A collection of tests. */
+export class Group {
+  public context: TestContext = new TestContext();
+  public results: Result[] = [];
+  public tests: Test[] = [];
+
+  constructor(public name: string) {}
+
+  /** Create a new test case that doesn't get run. */
+  skip(name: Test["name"], fn: Test["fn"]): void {
+    this.tests.push(new Test(name, fn, {skip: true}));
+  }
+
+  /** Create a new test case. */
+  test(name: Test["name"], fn: Test["fn"]): void {
+    this.tests.push(new Test(name, fn));
+  }
+
+  /** Run all the tests from this group and display their results. */
+  async run(): Promise<void> {
+    const results = await Promise.all(
+      this.tests.map(async (test) => test.run(this.context)),
+    );
+
+    console.log(
+      `# %c${this.name}`,
+      "font-weight: bold; text-decoration: underline;",
+    );
+    this.results = results;
+    for (const result of results) {
+      result.display();
+    }
+  }
+}
diff --git a/source/index.ts b/source/index.ts
new file mode 100644
index 0000000..58eaac1
--- /dev/null
+++ b/source/index.ts
@@ -0,0 +1,2 @@
+export * from "./group.js";
+export * from "./test.js";
diff --git a/source/test.ts b/source/test.ts
new file mode 100644
index 0000000..ddaa5af
--- /dev/null
+++ b/source/test.ts
@@ -0,0 +1,130 @@
+export class AssertionError extends Error {
+  public readonly actual: string;
+  public readonly expected: string;
+
+  constructor(message: string, actual: any, expected: any) {
+    super(message);
+
+    this.actual = JSON.stringify(actual);
+    this.expected = JSON.stringify(expected);
+  }
+}
+
+/** Test execution context with assertions. */
+export class TestContext {
+  /** Assert strict equality with `===`. */
+  equals<T>(actual: T, expected: T): void {
+    if (actual === expected) {
+      return;
+    }
+
+    throw new AssertionError("Failed equals assertion", actual, expected);
+  }
+}
+
+/** Special options for test cases. */
+export type TestOptions = {
+  skip?: boolean;
+};
+
+/** A test case. */
+export class Test {
+  constructor(
+    public name: string,
+    public fn: (test: TestContext) => Promise<void>,
+    public options?: TestOptions,
+  ) {}
+
+  /** Run the test and return its result. */
+  async run(test: TestContext): Promise<Result> {
+    const data: ResultData = {
+      duration: undefined,
+      error: undefined,
+      name: this.name,
+      status: "unknown",
+    };
+
+    const start = performance.now();
+    try {
+      if (this.options?.skip) {
+        data.status = "skipped";
+      } else {
+        await this.fn(test);
+        data.duration = performance.now() - start;
+        data.status = "passed";
+      }
+    } catch (_error: unknown) {
+      data.duration = performance.now() - start;
+      if (_error instanceof AssertionError) {
+        data.error = _error;
+      } else {
+        throw _error;
+      }
+
+      data.status = "failed";
+    }
+
+    return new Result(data);
+  }
+}
+
+/** Data created by a test case. */
+type ResultData = {
+  duration: number | undefined;
+  error: AssertionError | undefined;
+  name: string;
+  status: "failed" | "passed" | "skipped" | "unknown";
+};
+
+/** The result of a test case. */
+export class Result implements ResultData {
+  public duration: ResultData["duration"];
+  public error: ResultData["error"];
+  public name: ResultData["name"];
+  public status: ResultData["status"];
+
+  constructor(data: ResultData) {
+    this.duration = data.duration;
+    this.error = data.error;
+    this.name = data.name;
+    this.status = data.status;
+  }
+
+  /** Get the color associated with a result status. */
+  statusColor(): string {
+    if (this.status === "failed") {
+      return "red";
+    }
+
+    if (this.status === "passed") {
+      return "green";
+    }
+
+    if (this.status === "skipped") {
+      return "yellow";
+    }
+
+    return "white";
+  }
+
+  /** Print the result to the console. */
+  display(): void {
+    const bold = "font-weight: bold;";
+    const styles = [bold, `color: ${this.statusColor()}; ${bold}`];
+
+    let message = `- %c${this.name} %c${this.status}`;
+    if (this.duration !== undefined && this.duration > 1) {
+      message += ` %c${this.duration}ms`;
+      styles.push("color: white;");
+    }
+
+    if (this.error !== undefined) {
+      message += `\n  %c${this.error.message}`;
+      message += `\n  |   Actual: ${this.error.actual}`;
+      message += `\n  | Expected: ${this.error.expected}`;
+      styles.push("color: pink;");
+    }
+
+    console.log(message, ...styles);
+  }
+}
diff --git a/tests/example.ts b/tests/example.ts
new file mode 100644
index 0000000..9337d8e
--- /dev/null
+++ b/tests/example.ts
@@ -0,0 +1,13 @@
+import {setup} from "../build/index.js";
+
+const add = (a: number, b: number): number => a + b;
+
+void setup("add", async (group) => {
+  group.test("1 + 1", async (test) => {
+    test.equals(add(1, 1), 2);
+  });
+
+  group.test("2 + 2", async (test) => {
+    test.equals(add(2, 2), 5);
+  });
+});
diff --git a/tests/index.html b/tests/index.html
new file mode 100644
index 0000000..9a436ab
--- /dev/null
+++ b/tests/index.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<html lang="en">
+
+<head>
+  <meta charset="UTF-8">
+  <meta http-equiv="X-UA-Compatible" content="IE=edge">
+  <meta name="viewport" content="width=device-width, initial-scale=1.0">
+  <title>@holllo/test</title>
+  <style>
+    body {
+      background-color: #222;
+      color: #eee;
+    }
+  </style>
+</head>
+
+<body>
+  <script src="index.ts" type="module"></script>
+  <script src="example.ts" type="module"></script>
+</body>
+
+</html>
diff --git a/tests/index.ts b/tests/index.ts
new file mode 100644
index 0000000..c930e64
--- /dev/null
+++ b/tests/index.ts
@@ -0,0 +1,49 @@
+import {setup} from "../source/index.js";
+
+async function add(a: number, b: number): Promise<number> {
+  await new Promise<void>((resolve) => {
+    setTimeout(() => {
+      resolve();
+    }, Math.random() * 1000);
+  });
+
+  return a + b;
+}
+
+async function subtract(a: number, b: number): Promise<number> {
+  await new Promise<void>((resolve) => {
+    setTimeout(() => {
+      resolve();
+    }, Math.random() * 1000);
+  });
+
+  return a - b;
+}
+
+void setup("add", async (group) => {
+  group.test("add(1, 1) = 2", async (test) => {
+    test.equals(await add(1, 1), 2);
+  });
+
+  group.skip("add(1, 1) = 3", async (test) => {
+    test.equals(await add(1, 1), 3);
+  });
+
+  group.test("add(1, 1) = 4", async (test) => {
+    test.equals(await add(1, 1), 4);
+  });
+});
+
+void setup("subtract", async (group) => {
+  group.test("subtract(1, 1) = 0", async (test) => {
+    test.equals(await subtract(1, 1), 0);
+  });
+
+  group.skip("subtract(1, 1) = 1", async (test) => {
+    test.equals(await subtract(1, 1), 1);
+  });
+
+  group.test("subtract(1, 1) = 2", async (test) => {
+    test.equals(await subtract(1, 1), 2);
+  });
+});