Heuristics are fast-and-frugal strategies that guide action under uncertainty. We define what design heuristics are, why they matter under bounded rationality, and how they operate from framing through ideation to evaluation. A practical typology—form-, function-, behavior-, and system-based heuristics—helps designers move from open exploration to focused progress. The chapter also shows how to build an “adaptive toolbox” and demonstrates reflective use: naming, testing, refining, and combining heuristics with analysis.