Adolescence is a time of rapid growth caused by significant changes in hormone levels. For many, it is also a time of increased physical activity and sport that places a large demand on energy reserves. Exercise is known to cause perturbations in endocrine and metabolic systems in children and adolescents, yet careful characterization of these responses is only now being conducted. It does not appear that prepubertal youth have a different muscle composition than adults. However, these youth do have a lower anaerobic capacity and a greater reliance on aerobic metabolism during activity. Prepubertal adolescents may have an immature glucose regulatory system that influences glycemic regulation at the onset of moderate exercise. During heavy exercise, muscle and blood lactate levels are lower in children than in adults and there is a greater reliance on fat as fuel. The exercise intensity that causes maximal fat oxidation rate and the relative rate of fat oxidation decreases as adolescents develop through puberty. The mechanism for the attenuated lipid utilization with the advancement of puberty, and the impact that this may have on body composition, are unknown. Surprisingly, prepubertal adolescents have relatively high rates of exogenous glucose oxidation, perhaps because of their smaller endogenous carbohydrate reserves. Further study is needed to determine the optimal exogenous carbohydrate feeding regimen for peak performance in adolescence. Studies are also needed to determine whether physical activity, at an intensity targeted to maximize fat oxidation, help to lower body adiposity in overweight youth.