Heuristics are simple rules of thumb to find out or to discover. In this chapter, we offer an introduction into cognitive and decision making research on heuristics. In the first part, we provide a brief history of the term “heuristic”, and then trace back the eureka experiences made by a group of 20th-century researchers who set out, initially, to investigate everything but heuristics, but who ended up founding an entire research program on heuristics, namely the fast-and-frugal heuristics program (e.g., Gigerenzer, Todd, & the ABC Research Group, 1999). In the second part of this chapter, we then illustrate central concepts and methods of the fast-and-frugal heuristics program in more detail. We will show that the fast-and-frugal heuristics approach to judgment and decision making has grown, over the past three decades, from a widely ignored collection of ideas of some "rebels" to a full-blown, influential research program, motivating research and even policy in several disciplines and areas, ranging from psychology, philosophy, law, and medicine in the academic world to health care and education in the public, applied domain. In the third part, we present a few heuristics in more detail and review some exemplary work.