The history of the Institute for Laboratory Animal Research (ILAR) begins, as does all of laboratory animal science, with the ancient philosophers, anatomists, and physiologists whose work presaged the use of animals in biomedical research and the institutions that arose due to this use. Modern laboratory animal science and medicine began in the late 1940s and early 1950s as five Chicago-area institutions hired veterinarians to manage their animal facilities. Each of these men became instrumental in the founding of the organizations that collectively make up the laboratory animal science and medicine organizations. Nathan Brewer, one of the "Chicago five," was particularly influential in the founding of ILAR. His boss at the University of Chicago, Dr. Paul Weiss, a member of the National Academy of Sciences (NAS), asked him to help establish a committee with the stated purpose of preparing recommendations to the NAS to develop an office to obtain information on sources of supply for research animals. This office became ILAR, and Brewer was chairman of its first report on the diseases of laboratory animals. He was also a founding diplomat and first president of the American College of Laboratory Animal Medicine. This history recognizes the thoughtful and energetic contributions of scientists and veterinarians to ILAR. It provides a 50-year overview of the programs and reports of ILAR and highlights examples where these reports have been adopted by scientists and federal agencies and incorporated into national laws and policies governing the use of animals in research both in the United States and in other countries.