A back-end testing developer conducts tests on the server-side of applications to ensure functionality, performance, and reliability. They write and execute test cases, identify and document bugs, and verify bug fixes. They work closely with developers to understand system requirements and design appropriate test plans. They use various tools and languages like SQL, Java, Python, and testing frameworks. They also test APIs, databases, data processing, and server optimizations. Their ultimate goal is to identify any issues or weaknesses in the back-end structure before the product reaches the end-user, ensuring a smooth and efficient user experience.