The aging of the population of the United States and a concern for the well-being of older people have hastened the emergence of measures of functional health. Among these, measures of basic activities of daily living, mobility, and instrumental activities of daily living have been particularly useful and are now widely available. Many are defined in similar terms and are built into available comprehensive instruments. Although studies of reliability and validity continue to be needed, especially of predictive validity, there is documented evidence that these measures of self-maintaining function can be reliably used in clinical evaluations as well as in program evaluations and in planning. Current scientific evidence indicates that evaluation by these measures helps to identify problems that require treatment or care. Such evaluation also produces useful information about prognosis and is important in monitoring the health and illness of elderly people.