Although it is self-evident that the study of human nutrition has as its goal the optimal nutrition of man, the nutritional status of the body best suited to optimal performance, i.e., optimal nutrition, has unfortunately not yet been satisfactorily defined. Body composition of animals may be measured by direct chemical analysis and correlated with dietary intake and with the various aspects of performance; studies of body composition of living man, on the other hand, must rely on indirect measure ments. The following two reports, which give an account of the current status of the attack on the difficult task of measuring body composition in living man, are sponsored by the Committee on Nutrition to call attention to the resurgence of effort in this field during recent years. A fuller knowledge of the gross composition of the human body and its relation to preceding diet will constitute a significant step towards realization of the ultimate goal of nutritional science. Even then, a particular body composition will be of importance primarily in terms of functional performance. The availability of newer techniques should do much to stimulate physicians and nutritionists in defining body composition as an essential step in arriving at a more exact definition of optimal nutrition.