The early nineteenth century witnessed the first concerted application of chemical techniques to the investigation of biologic materials. Milk was chemically analyzed, von Liebig discovered the chemical contrast between potassium-rich cells and sodium-rich tissue fluids, and Carl Schmidt analyzed blood serum in great detail. Soon these methods were applied to the study of the entire human body. The advent of isotopic dilution techniques in the 1930's and 1940's brought about a renewed interest in body composition. More recently these and other techniques have been used in an attempt to study the nutritional aspects of body composition. It is the purpose of this report to present the various methods now in use and to discuss the virtues and limitations of each. DIRECT CHEMICAL ANALYSIS Whole Body During the past century a number of human carcasses have been analyzed for water, fat, nitrogen, sodium, potassium, chloride, calcium, magnesium, and phosphorus. These include the fetus, newborn, and adult. The few analyses done on children have included subjects suffering from conditions known to profoundly affect water and electrolyte metabolism. Data for total body content in the new-born and adult are presented in Table I, both on a fresh-weight and a fat-free fresh-weight basis. The latter basis for expressing values avoids discrepancies that arise because of alterations in fat content. It is evident that significant changes in body composition occur during growth. Water, sodium, and chloride contents decline, while contents of nitrogen, calcium, potassium, magnesium and phosphorus all increase. In these data, body fat content is not strikingly different in the two age groups.