The way we feel about ourselves is important in every domain of life, and we can almost certainly identify when we feel on top of the world about something. In this case, we are enjoying the benefits of high self-esteem, a concept that has been branded a 'social vaccine' that can empower individuals and make them more productive and confident in their own abilities. The self is one of the most widely examined psychological constructs in contemporary society. Historically, both the academic literature and the popular media presuppose that every person has a sense of self, and the self is a social phenomenon (Cooley, 1902; Mead, 1934; James, 1950). Who we are is directly related to our relationships with others. Our interactions with friends, family and peers provide information that guides our understanding of how and why we think, feel and behave the way we do. This is often termed global self-worth, and researchers and scientists have often conceptualized self-worth as an evaluation or collection of different but related components that reflect how we feel about particular facets of our lives, such as academic self-esteem, social selfesteem and body esteem (Sonstroem, 1998). Indeed, self-esteem is often seen to be one of the most important indicators of psychological well-being, and, in many instances, enhanced self-esteem resulting from physical activity is one of the main benefits put forward by those individuals or agencies promoting regular exercise or within physical education in schools. Subsequently, exercise scientists have taken an interest in the role that physical activity and exercise can play in enhancing the self. Involvement in regular physical activity may improve our skills, knowledge, fitness and health and contribute to our social lives, all of which are linked to enhanced self-perception (Fox, 1997). This, in turn, may transfer to more favourable views about the self, leading to an improved sense of well-being. Exercise psychologists should have a particular interest in the role that physical activity plays in self-esteem. This can then be transferred to practice, where the well-being of the individuals we work with can be enhanced. This topic forms the basis of this chapter. The aims of this chapter are threefold: the first is to describe the relationships between physical activity and self-esteem reported in the literature. A second aim is to evaluate the conceptual exercise-self-esteem model of Sonstroem and Morgan (1989) and evidence that has examined this model. The third aim of the chapter is to develop an understanding of the methods available to assess self-esteem in relation to physical activity.