Tissues transferred between genetically nonidentical individuals are destroyed through a process known broadly as rejection. It has been apparent throughout most of medical history that these tissues could provide relief from disease if they were not rejected. Thus, the field of transplantation has grown in tandem with the understanding of the biology of rejection and, to the extent that rejection is an immune-mediated process, of the immune system in general. This close relationship between immunological science and clinical transplantation has fueled remarkable progress in our understanding of immune function and of the fundamental nature of our existence as individuals. The components of the immune system that have been defined in this context are now widely recognized not only for their importance in graft rejection but also for their roles in infection control, shock, tumor growth, autoimmune disease, and the systemic response to trauma. As such, the understanding of immunology that has been born of the study of transplantation has become key to the thorough understanding of the biology of modern medicine and surgery.