Heart transplantation remains one of the major scientific achievements of twentieth century medicine. During the past four decades, it has evolved from an unproven experimental surgical technique to the most effective form of therapy for refractory end-stage heart disease. It has captured the public's imagination and expanded our understanding of fundamental immunologic mechanisms that are responsible for cellular and humorally-mediated immunity. Despite its successes, many clinical and scientific problems remain. One or more bouts of acute cellular or humoral (vascular) rejection will occur...