This dissertation traces the origin and the development of gerontology, the science of aging, in the United States and the United Kingdom. I argue that gerontology began to be formed as a multidisciplinary scientific field in the two countries from the 1900s to the 1950s. Unlike earlier scholars who had thought that the aging of the whole body was caused by the inevitable decline of an unknown critical factor, such as "vital heat," gerontologists of the twentieth century conceived aging as a contigent phenomenon whose rate and mode differed in distinct portions of the body. They also introduced systematic experimental approaches in their investigation which had seldom been employed in the study of aging before the twentieth century. Furthermore, with these new ideas and methodologies, gerontologists established their research field in which scholars from diverse disciplines could work in a cooperative manner, including biologists, physicians, psychologists, and social scientists. Amid the Great Depression, which threatened the very survival of the elderly, these multidisciplinary scholars formed professional societies and research institutes for more organized study of aging. But gerontology followed different paths of development in America and Britain due to their distinctive political and cultural conditions, academic traditions, and leading scholars' social and academic status. While British scientists of aging were struggling with various problems related to funding, professional recognition, and the recruitment of scholars interested in aging, American gerontologists came to have relatively ample and stable sources of financial support and an expanding network of national and local organizations. By analyzing this difference and tracing the beginnings of the new concepts and approaches, this dissertation aims at explaining the birth of a multidisciplinary scientific field within historical contexts.