A lesson from a course on medical software development lays out the core principles of software testing. The instructor distinguishes testing from validation and verification, explaining why bug-finding is a continuous process rather than a one-time checkpoint. The lecture compares white box testing, which examines internal code logic, with black box testing, which checks outputs against inputs without inspecting the code, then walks through testing levels from unit tests on individual functions up through integration and full system testing. Particular attention goes to the medical domain, where software errors carry higher stakes, and the lecture covers strategies like reducing code complexity and running interactive user testing to catch problems automated checks miss. At sixteen minutes, it stays tightly focused on definitions and methodology rather than case studies, functioning as a practical primer for anyone building or evaluating software meant for clinical use.