Aging is characterized by deleterious changes in body composition and in fat distribution. The mechanisms that determine the aging-associated changes in body composition are not well defined, but the evidence suggests that the loss of fat-free mass is at least partially attributable to physical inactivity. The increase in fat mass may be the result of alterations in fatty acid metabolism. Indeed, fat oxidation is decreased in elderly individuals in several physiological conditions: a) at rest, b) during exercise, and c) in response to meal ingestion (after weight loss). These defects are related in part to loss of fat-free mass, but may also be the consequence of estrogen loss (in women) and/or a decrease in the intrinsic capacity of muscle for fat oxidation, and are amenable to partial correction by exercise training. Special emphasis should be placed in future studies upon the role of steroid hormone in the regulation of fatty acid metabolism in elderly individuals (especially women), as well as therapeutic interventions that may increase the quantity of the fat-free mass and/or fat oxidation.