This review examines the evidence that the level of physical activity (PA) or total energy expenditure during adolescence affects body adiposity in the obese and nonobese adolescent population. Several cross-sectional studies suggested that obese children were less physically active than their nonobese peers, but there was no consistent difference in the total energy expenditure. The likelihood that infants of obese mothers become obese at age 1 year is greater if their total energy expenditure (using the doubly labeled water technique) is lower at age 3 months. Many interventional studies in the general adolescent population show a small (1-3% body fat) reduction in adiposity as a result of physical training. It appears, though, that programs longer than one year are more efficacious than shorter programs. Lifestyle activities (e.g., walking to and from school) appear to have a more lasting effect than regimented activities (e.g., calisthenics or jogging).