As we come to the close of the second millenium, it is inte resting to note that almost everything we know about the transplantation of tissues and organs has come about in the 20th century. Alexis Carrel received the Nobel Prize in medicine in 1912 for his work on vascular suture s and the transplantation of blood vessels and organs. He and others showed that an allogeneic graft could function for a time but then would fail. The antigenic basis of tumor transplants was established based on experiments in mice [1] and led to the recognition of the H2 transplantation antigen system [2]. Medawar clearly established the immunologic basis for graft rejection and described neonatal tolerance in mice [3]. Thus, at the end of Wo rld War II, the stage was set for the development of hematopoietic cell transplantation.