What are test cases and how do you write a good one?

A test case is a set of conditions or variables under which a tester will determine whether a system or part of an application is working correctly. A good test case should be clear, concise, and cover a single scenario. It typically includes a unique ID, a description, pre‑conditions, input data, expected results, and post‑conditions. It should be independent, repeatable, and traceable to requirements. Good test cases also consider edge cases, error conditions, and performance aspects, and they are written in a way that anyone can understand and execute them without ambiguity. When writing test cases, start by mapping each requirement to one or more test cases to ensure coverage. Use clear, unambiguous language and avoid assumptions about the system state. Include both positive and negative test scenarios to validate correct behavior and error handling. Document the expected outcome precisely, including any side effects or state changes. Review test cases with peers to catch missing conditions or unrealistic expectations. Finally, maintain a test case repository that tracks status, defects found, and coverage metrics, enabling continuous improvement of the testing process.