Advancing adult age is associated with profound changes in body composition, the principal component of which is a decrease in skeletal muscle mass. This age-related loss in skeletal muscle has been referred to as sarcopenia. Agerelated reduction in muscle is a direct cause of the age-related decrease in muscle strength. Muscle mass (not function) appears to be the major determinant of the age- and sex-related differences in strength. This relationship is independent of muscle location (upper vs lower extremities) and function (extension vs flexion). Reduced muscle strength in the elderly is a major cause for their increased prevalence of disability. With advancing age and extremely low activity levels seen in the very old, muscle strength is a critical component of walking ability. The high prevalence of falls among the institutionalized elderly may be a consequence of their lower muscle strength. Daily energy expenditure declines progressively throughout adult life. In sedentary individuals, the main determinant of energy expenditure is fat-free mass, which declines by about 15% between the third and eighth decade of life, contributing to a lower basal metabolic rate in the elderly. Data indicate that preservation of muscle mass and prevention of sarcopenia can help prevent the decrease in metabolic rate. In addition to its role in energy metabolism, skeletal muscle and its age-related decline may contribute to such age-associated changes as reduction in bone density, insulin sensitivity, and aerobic capacity. For these reasons, strategies for preservation of muscle mass with advancing age and for increasing muscle mass and strength in the previously sedentary elderly may be an important way to increase functional independence and decrease the prevalence of many age-associated chronic diseases.