Gerontology is the study of aging and older adults. The science of gerontology has evolved as longevity has improved. Researchers in this field are diverse and are trained in areas such as physiology, social science, psychology, public health, and policy. A more complete definition of gerontology includes all of the following: Scientific studies of processes associated with the bodily changes from middle age through later life; Multidisciplinary investigation of societal changes resulting from an aging population and ranging from the humanities (e.g., history, philosophy, literature) to economics; and Applications of this knowledge to policies and programs.