Anatomists showed many years ago that some organs grow at rates that differ from that of the body as a whole. In recent decades we have witnessed the development of techniques that permit the assessment of certain features of body composition in the living subject. Humans rank among the fattest of mammals, and it is this fact, among others, which has stimulated the development of methods for estimating the relative proportions of lean and fat in our species; assays for total body Ca, P, and N have also been developed. These methods make it possible to define the contribution of each component to the adolescent growth process. Body composition data can also help in estimating certain nutritional requirements for growth. Although long a favorite of nutritionists, the metabolic balance method is not equal to this task (Wallace, 1959; Forbes, 1973, 1983), and one can look forward to more realistic estimates based on these newer techniques.