Aging is associated with changes in body composition and energy and protein metabolism that are due both to the direct effects of aging and to the effect of age-related diseases. We have recently differentiated these changes under three categories: wasting, cachexia, and sarcopenia. We have defined wasting as unintentional loss of weight, including both fat and fat-free compartments. Experience in the HIV epidemic suggests that wasting is driven largely by inadequate dietary intake. Cachexia, on the other hand, refers to loss of fat-free mass, and especially body cell mass, but with little or no weight loss. The metabolic hallmarks of cachexia are hypermetabolism and hypercatabolism, driven by inflammatory cytokine-mediated acute phase responses. Finally, sarcopenia refers to loss of muscle mass specifically, and seems to be an intrinsic age-related condition. In the elderly, wasting as defined here is at the extreme end of the spectrum, but generally develops in the setting of pre-existing sarcopenia and cachexia. The challenges before us now are to better define these conditions, establish guidelines for their recognition, and develop better methods for intervening when appropriate.