Beliefs are, or at least appear to be, integral to cognition and action. Though there are scarcely features of human psychology more intuitive to their bearers, beliefs are surprisingly elusive targets of study. In this chapter, we consider some perennial questions about beliefs and suggest that some clarity might be achieved by viewing beliefs through the lens of cognitive psychology. We discuss psychological findings and evolutionary considerations which seem to imply that the mind is not designed to form true beliefs, but beliefs that are instrumentally useful. This issue is redolent of debates over whether people are rational or irrational and whether beliefs aim at truth or serve other psychological functions. We survey a series of practical tradeoffs and computational constraints that limit the attainment of true beliefs, and which may be responsible for apparent irrationality. Additionally, the origin of false or irrational-seeming beliefs may be inadequately specified by behavioral data, which implies that a deeper understanding of processes and prior knowledge inside the head is essential for a science of beliefs. We conclude by noting that a view of irrational beliefs as the result of prior knowledge, rather than irrational processes, may have optimistic implications for improving people’s beliefs.