The effects of disability on an aging population's health and welfare are an important issue in gerontological research. The rapid growth of the elderly population and increases in longevity have led to an ongoing debate about whether longer lives can be matched by longer active lives that are free from disability. After a detailed review of current disability literature, the authors discuss the impact of disability in the elderly, defining disability and reviewing three classes of disability--physical, mental, and social. Both subjective and objective disability measures are described, and disability trends and prevalence rates are reviewed and compared cross culturally, by gender, by age, and over time. The path from chronic disease to disability is described and the consequences of living with disability are discussed in terms of family burdens and the increased need for medical care.