Top-down design and modular development are techniques used by computer programmers to break down large, complex problems into smaller, more manageable parts. Top-down design involves starting with a clear statement of the overall problem and then systematically breaking it into sub-problems, while modular development involves developing individual software modules separately and then combining them into a solution. These techniques make projects more manageable, faster to complete, higher quality, easier to debug, and promote code reusability.