To reach desirable outcomes within a population, we regularly rely on policies: population-based interventions aiming to optimize outcomes through guiding behaviors and decisions. Policies are developed for use by anyone from individuals and families to large groups and major organizations. While often considered a nebulous system of complex processes used by governments, policies are often very simple guidelines and can relate to very basic, everyday activities. This chapter introduces core concepts of policy, how it is distinct from – yet overlaps with – laws and regulations, the historical development of policy, and the making of policies as an area of scientific interest. This chapter sets the tone for the book, framing key terms and topics critical to maximizing the value of every section through the contributions of psychological and behavioral sciences to policy.