JUnit is Java's most widely adopted testing framework, enabling developers to write and execute unit tests with annotations, assertions, and lifecycle management. JUnit 5 (Jupiter) introduced a modular architecture, extensibility through custom extensions, parameterized tests, dynamic tests, and enhanced conditional execution—moving beyond JUnit 4's rigid @RunWith model. It supports behavior-driven testing patterns, mocking integration (Mockito), parallel execution, and IDE/build tool integration, making it essential for test-driven development (TDD), continuous integration pipelines, and ensuring code correctness. Understanding JUnit's lifecycle hooks, assertion methods, and extension model allows practitioners to write maintainable, isolated tests that catch regressions early and document expected behavior through executable specifications.