Unit testing is a critical software testing methodology focusing on verifying the behavior of individual functional units of code to ensure they perform as intended. It allows developers to catch bugs early, validate logic, and maintain high-quality code, often integrated into CI/CD pipelines for continuous testing as new code is committed. The methodology emphasizes independent, modular code design, and while it excels in testing isolated functions, it does not provide a comprehensive system evaluation. Advanced techniques like mocking and test-driven development (TDD) enhance unit testing by simulating interactions with external dependencies and prioritizing test creation before code development. Automated unit testing in CI/CD pipelines enhances efficiency by running tests in parallel, which is crucial for large projects. However, challenges such as flaky tests and maintaining up-to-date test suites persist, requiring developers to actively manage and update their tests. Overall, unit testing is an indispensable tool for ensuring code quality and functionality throughout the development lifecycle.