Undernutrition is common in the elderly, particularly those in nursing homes and other institutions. It is associated with substantial adverse effects. The age-associated physiologic reduction in appetite and food intake, which has been termed "the anorexia of aging," contributes to the development of pathologic anorexia and undernutrition. This article reviews age-related changes to appetite, food intake, and body composition; undernutrition in the elderly; and the factors contributing to physiologic and pathologic anorexia and undernutrition.