The idea that more information and more computation yield better decisions has long shaped our vision of rationality. Yet humans and other animals typically rely on simple heuristics or rules of thumb to solve adaptive problems, focusing on one or a few important cues and ignoring the rest, and shortcutting computation rather than striving for as much as possible. In this book, the authors argue that in an uncertain world, more information and computation are not always better, and instead ask when, and why, less can be more. The answers to these questions constitute the idea of ecological rationality, as explored in the chapters in this book: how people can be effective decision makers by using simple heuristics that fit well to the structure of their environment. When people wield the right tool from the mind's adaptive toolbox for a particular situation, they can make good choices with little information or computation-enabling simple strategies to excel by exploiting the reliable patterns in the world to do some of the work. Heuristics are not good or bad, "biased" or "unbiased," on their own, but only in relation to the setting in which they are used. The authors show heuristics and environments fitting together to produce good decisions in domains including sports competitions, the search for a parking space, business group meetings, and doctor/patient interactions. The message of Ecological Rationality is to study mind and environment in tandem. Intelligence is not only in the mind but also in the world, captured in the structures of information inherent in our physical, biological, social, and cultural surroundings.