This article has set out to provide basic knowledge about body composition in athletic and nonathletic adolescents and young adults and to provide the practicing physician with methods of making body composition assessment. We suggest the physician approach the adolescent athlete who requests information about body composition in the following way: 1. Calculate the ideal body weight. 2. Estimate the percentage of body fat, realizing the errors associated with each method. If a body composition laboratory is available, use that equipment. In the absence of this equipment, we recommend the equations of Slaughter et al, given earlier. 3. The athlete should be given a range of percentage of body fat values measured in other athletes of the same gender and sport. Health and performance should be monitored as the athlete attempts to achieve or maintain body composition in this range. 4. If the athlete has an interest in altering body composition, then recommend the athlete seek the advice of a professional who has expertise in nutrition and physiology.