he changes in composition of the aging T human body is of both gerontologic and geriatric interest and reflects genetic factors and environmental factors such as physical activity, nutrition, and disease, as well as the normal aging processes. In the study of normal aging, that is, aging per se, long-term trends in body compartments are important. In the field of clinical geriatrics, body composition is also of great interest in the study of obesity, malignant disease, and clinical pharmacology and biochemistry. Changes of body cell mass, body fat, and body water occur frequently in both disease and aging. These changes in body compartments may alter the pharmacokinetic and pharmacodynamic properties of drugs used in old age. The amount of body fat and the amount of body cell mass are important parameters when judging the net effect of energy intake and expenditure. Thus, examination of body composition adds information to an overall picture of nutrition of the elderly. Such overall knowledge also requires data on actual dietary habits of elderly populations in different countries. Not only intakes of energy, nutrients, and food items are important, but also distribution and composition data on meals and data on medical, psychologic, and socioeconomic conditions are prerequisites for the analysis of the relationship between requirements and actual intake. The need for broad epidemiologic approaches to these questions is obvious. Concepts The frequently used concept of “lean body mass” has two meanings, namely, body weight minus neutral fat, but including some lipids such as phospholipids, the exact amount of which is unknown. Lean body mass is, according to this meaning, similar to, but not identical with, fat-free body mass. Although lean body mass has a relatively constant specific gravity, from a biologic point of view it is a heterogenous body component with varying energy consumption, since it also comprises-apart from body cel I massfat-f ree ext race1 I u lar solids and extracellular water. Furthermore, since the water content of adipose tissue may show great variation and is also difficult to estimate, the definition of lean body mass should be body weight minus body fat, that is, fat-free mass. Many workers, nowadays, however, avoid the concept of a lean body mass and fat-free mass, since the body is not a simple two-compartment system. Moore and collaborators’ many years ago suggested that the concept of body cell mass would provide a better estimate than lean body mass of metabolically active tissue. Many workers now prefer to divide body weight into body cell mass, extracellular water, body fat, and fat-free extracellular solids that are present primarily in bone and connective tissuea2 For a review of the concepts of body compartments, see references 1, 3, 4, and 5.