This chapter describes Cialdini's six principles of persuasion and reviews studies in which they have been tested in laboratory or real-life settings. It provides examples of intervention strategies that aim to implement those principles in the context of physical activity and sport. The chapter discusses similarities and differences among the six principles of persuasion, reveals potential implications for the promotion of physical activity participation and suggests directions for future research. The principles of social influence identified by Cialdini have been examined extensively in the context of consumer behavior, but less so in the context of physical activity or sport. In examining social influence effects, it is also important to keep in mind that there are conceptual similarities among the constructs captured by the six principles of social influence. For instance, a commonality of descriptive norms, authority, and liking is that they all refer to communicators' characteristics.