This document traces theoretical developments in the psychology of aging during the last 50 years. The concept of theory is discussed as well as the bringing together of theories to form a model. After summarizing the early beginnings of American interest in aging, the work of major theoreticians is explored including Hall (senescence), Thorndike (learning theory), Miles (testing), Terman (testing), and Cowdry (modern gerontology). The founding of relevant organizations is also described, including the gerontology unit within the National Institute of Health; the Gerontological Society of America; and Division 20 of the American Psychological Association, formerly entitled Maturity and Old Age, and now known as Adult Development and Aging. (AG)