Over the past 40 years, the assessment and scientific study of capacity in older adults has emerged as a distinct field of clinical and research activity for psychologists. This new field reflects the convergence of several trends: the aging of American society, the growing incidence and prevalence of dementia, and the patient rights, deinstitutionalization, and disability rights movements. Because of these forces, capacity issues now permeate the fabric of everyday life, whether in the form of guardianship petitions, questions of capacity to consent to treatment, the ability to make a new will, or participation in human research. In seeking to resolve these issues, families, clinicians, and legal professionals increasingly turn to psychologists to assess a capacity and to provide empirically supported judgments that properly balance autonomy and protection for the individual. Psychologists have taken a leading role in the development of functional assessment instruments that measure important aspects of the capacity construct. In addition, psychology has been a major contributor to the scientific study of capacity. In collaboration with colleagues from medicine and law, psychologists have articulated crucial theoretical frameworks that integrate legal, clinical, and ethical dimensions of the capacity problem. This article focuses on the evolution of theory, law, science, and practice in the evaluation of capacity in older adults and its recent culmination in a series of interdisciplinary handbooks sponsored by the American Psychological Association and the American Bar Association.