This volume presents a broad spectrum of research on the Cayo Santiago macaques, a unique free-ranging colony of rhesus macaques in Puerto Rico. It includes thirteen scientific studies on the behavior and biology of the Cayo Santiago macaques, as well as a detailed history of the colony and a complete bibliography of over 260 scientific publications based on work at Cayo Santiago from 1938 through 1984. The chapters represent examples of both short- and long-term research conducted on the island over the past several years. Some are reviews, providing a synopsis of complex longitudinal studies of behavior, vocal communication, functional morphology, genetics, and population dynamics. Others document the results of opportunistic studies of behavior or biological surveys. The chapters cover a broad range of topics, but all share a common dependence on the detailed life history and genealogical data which make the Cayo Santiago macaque colony an important international research resource.