It is commonly believed that participation in structured sport programs leads to positive experiences and beneficial developmental outcomes for children and adolescents. For instance, proponents of organized sport cite that participation can help build self-esteem, promote sportspersonship, encourage a valuing of physical activity, and provide a sense of enjoyment and well being. While there is some empirical support for the relationships between sport participation and these outcomes, research shows that children and youth can also experience a number of negative outcomes, such as high levels of stress, burnout, and low self-esteem as a result of their participation. Thus, any positive effects of sport participation on psychosocial development and well being cannot be assumed to be an automatic consequence of participation--simply signing up to play will not guarantee positive results. Rather, the impact of sport on youth will ultimately depend on their experiences "while" participating. This study aims to better understand "how" participation in sport leads to more or less positive outcomes for youth by exploring the link between athletes' psychological need satisfaction and their well being. Specifically, the authors test whether changes in perceived competence, autonomy, and relatedness over a competitive season were predictive of changes in athletes' well being in their global self-esteem and burnout level in sport. (Contains 1 table and 2 notes.)