Test-Driven Development (TDD) is a software development practice where tests are written before the code that makes them pass, popularized by Kent Beck in the late 1990s as part of Extreme Programming. TDD follows the red-green-refactor cycle: write a failing test (red), write minimal code to make it pass (green), then improve the code structure (refactor). This approach transforms testing from an afterthought into a design tool that drives cleaner, more maintainable code. The practice emphasizes small, incremental steps and continuous validation, providing instant feedback and catching issues at the earliest possible moment.