Frailty is a new and emerging syndrome in the field of geriatrics. The study of frailty may provide an explanation for the downward spiral of many elderly patients after an acute illness and hospitalization. The fact that frailty is not present in all elderly persons suggests that it is associated with aging but not an inevitable process of aging and may be prevented or treated. The purpose of this article is to review what is known about frailty, including the definition, epidemiology, and pathophysiology, and to examine potential areas of future research.