Modifiable human behavior has a key influence on health, healthcare and health outcomes. Behavioral science allows the application of theory, evidence and systematic methods to understand the nature of these behaviors and their influences, in order to help improve population health. This chapter introduces some frequently used theories of behavior change and evaluates their ability to explain health-related behaviors. It discusses how theory can be used to develop behavior change interventions for prevention, disease management and healthcare practice, and introduces some frameworks aimed at integrating and simplifying theoretical approaches, methods and reporting. Finally, this chapter outlines work aiming to develop consistent terminologies for theoretical concepts and behavior change techniques, to support communication and collaboration to advance behavior change theory.