How do you test asynchronous code?

Testing async code requires test frameworks that support async operations. JavaScript/Jest: return a Promise or use async/await: test("fetches user", async () => { const user = await fetchUser(1); expect(user.name).toBe("Alice"); });. Use done callback for callback-style async. Mock async functions: jest.fn().mockResolvedValue(data). C# (xUnit/NUnit): async Task test methods are natively supported: [Fact] public async Task TestAsync() { var result = await service.GetAsync(); Assert.Equal("expected", result); }. Python (pytest-asyncio): @pytest.mark.asyncio async def test_fetch(): result = await fetch_data(); assert result == expected. Common pitfalls: forgetting to await (test passes even if async code throws); not handling rejection (unhandled promise rejections may not fail tests in some frameworks); real timers causing timeouts. Use fake timers (jest.useFakeTimers()) for time-dependent async code. waitFor utility (React Testing Library) waits for assertions to pass in async UI tests.