Unintentional weight loss, or the involuntary decline in total body weight over time, is common among elderly people who live at home. Weight loss in elderly people can have a deleterious effect on the ability to function and on quality of life and is associated with an increase in mortality over a 12-month period. A variety of physical, psychological and social conditions, along with age-related changes, can lead to weight loss, but there may be no identifiable cause in up to one-quarter of patients. We review the incidence and prevalence of weight loss in elderly patients, its impact on morbidity and mortality, the common causes of unintentional weight loss and a clinical approach to diagnosis. Screening tools to detect malnutrition are highlighted, and nonpharmacologic and pharmacologic strategies to minimize or reverse weight loss in older adults are discussed.