Young athletes of both sexes grow as well as nonathletes. The experience of athletic training and competition does not harm the physical growth and development of the youngster. The young, trained athlete has generally a lesser percentage of body weight as fat. Maturity relationships are not consistent across sports. Male athletes more often than not tend to be advanced maturationally compared with nonathletes. On the other hand, female athletes tend to be delayed in maturity, except for swimmers. Maturity-associated variation in size and body composition is thus a significant factor in comparing athletes and nonathletes, especially during the circumpuberal years.