This project addresses the current epidemic in obesity and physical inactivity among adolescents in the United States. The goal of the research is to develop effective ways of encouraging adolescents to become and remain physically active. In particular, this project is concerned with identifying adolescents who are reluctant to exercise because they have a high sensitivity to unpleasant feelings while exercising at higher intensities. Once identified, these adolescents can be targeted with an intervention that is designed to teach them to exercise at an intensity level that will generate pleasant feelings and therefore make it more likely that they will seek out opportunities to be physically active. The specific aims of this study are to: 1) evaluate the impact of a novel intervention delivered via school-based physical education (PE) on adolescents who have a high sensitivity to exercise-induced negative affect; 2) determine whether adolescents' tendency to feel uncomfortable during exercise is a stable trait that persists even in the face of an intervention; and 3) compare and contrast three alternative methods of measuring adolescents' sensitivity to exercise-induced affect.