It is generally believed that individuals encounter functional decline associated with aging. Traditional definitions of function focus on physiologic organ and body system performance or on the ability to perform activities of daily living. The authors propose to expand the concept of function to include adapting to one's environment and the changes brought on by aging in all spheres of life—physiologic, psychologic, and social. Data from a longitudinal study of aging with spinal cord injury highlight the interrelationship of medical and nonmedical variables in predicting outcomes in these life spheres. Application of a global thesis of function to the general aging population is described.