Theories of psychosocial influence identify important psychological factors that influence exercise behavior and outline the mechanisms and processes by which these factors exert their influence. This chapter summarizes the psychosocial influences on exercise behavior using two popular theoretical approaches from social psychology: social cognitive theory and the theories of reasoned action and planned behavior. Synthesizing research from these theories has identified self-efficacy, attitudes, and perceived behavioral control as key factors influencing exercise intentions and behavior and has labeled them as targets for interventions to promote exercise behavior. The effects of other factors, such as personality and implicit variables, and features of the constructs, such as intention stability, are also reviewed. Implementation intentions are identified as important means to address the intention–behavior gap. The implications of the research for the development of interventions to most effectively promote exercise participation are considered.