White-box testing is a software testing method that examines the internal structures or workings of an application. Testers have access to the source code and design documents, allowing them to create test cases based on the program's logic and structure. This approach helps identify vulnerabilities, errors, and areas for optimization. In contrast to black-box testing, which focuses on input and output without knowledge of the internal workings, white-box testing provides a deeper understanding of how the software operates. It is often used in unit testing, integration testing, and security testing to ensure that all code paths are tested effectively.