This chapter shows how simple heuristics can be an essential tool for navigating the complexities and vagaries of social environments. The research program on the nature of social rationality presented here can be summarized by the following theses: As perceived by the human mind, the social world (Umwelt) is complex, but not necessarily more complex than the nonsocial world. However complex the social world may be, its complexity does not require cognitive complexity; rather, it entails conditions that make simple heuristics indispensible, such as intractability, multiple competing goals, and incommensurable reasons. Much of reasoning and decision making occurring in human and animal social environments can be modeled in terms of simple heuristics. Although simple heuristics forgo extensive information search and complex calculations, they can be as accurate and even more accurate than more complex strategies and/or can be used to reach other goals that are valued in social environments (e.g., transparency, fairness, speed). Heuristics can be simultaneously successful and simple by coopting evolved capacities. The capacities themselves can represent complex adaptive specializations (e.g., memory, movement tracking). Simple heuristics per se are neither rational nor irrational. Their rationality is ecological. That is, their performance depends on the match between the architecture of the heuristic and the structure of the environment in which it is used. The heuristics' simplicity inoculates them against overfitting and enables them to achieve robust performance given small samples of information. Simple heuristics can model adaptive decision making both in games against nature and in social games. There is no social intelligence distinct from nonsocial intelligence. Simple heuristics are tools of moderate generalizability. Some can be used only in games against nature, whereas others are restricted to social games. Still other heuristics can be applied in both types of games. Shedding light on the adaptive toolbox of simple heuristics used to navigate social environments, and characterizing their strengths and weaknesses, can help us design environments and/or heuristics in ways that improve public welfare.