Reinforcement is central to behavioral theory and behavior modification. This chapter examines the principles of behavioral theory and behavior modification, for which reinforcement is an important element. It explores cognitive evaluation theory, which addresses the impact of external rewards on intrinsic motivation. The chapter describes research that examines the impact of teacher feedback and external rewards on children's motor skill performance, self-perceptions, and prosocial behaviors in physical activity settings. It presents ways to effectively use reinforcements and punishments in physical education, based on the theories and research. Important to behavior modification are the various schedules of reinforcement, or the timing and frequency with which a person is reinforced, and how these affect the learning and maintenance of behaviors. Two aspects of reinforcement and motor skill performance have been investigated in research: verbal feedback and reinforcement as part of behavioral coaching programs based on behavior modification principles.