A good unit test is one that gives you fast, reliable feedback about a small, well-defined piece of code. You want it to test only one “unit” of behavior in isolation (for example, a single function or method), with no hidden dependencies like databases or external services. A strong test is deterministic — it always passes or fails consistently — and it has clear, readable assertions that show exactly what’s expected. It should follow the “Arrange-Act-Assert” pattern so you set up inputs, run the code, and check outputs in a way that’s easy to understand. Good unit tests are also small, run quickly, and cover both common cases and edge cases. Importantly, you should be able to change unrelated parts of your codebase without breaking them, which means they’re focused on behavior, not implementation details.