The science of transferring a graft from one part of the body to another or from one individual to another. The graft may consist of an organ, tissue, or cells. If donor and recipient are the same individual, the graft is autologous. If donor and recipient are genetically identical it is syngeneic. If donor and recipient are any other same-species individuals, the graft is allogeneic. If the donor and recipient are of different species, it is called xenogeneic. In theory, virtually any tissue or organ can be transplanted. The principal technical problems have been defined and, in general, overcome. Remaining major problems concern the safety of methods used to prevent graft rejection and the procurement of adequate numbers of donor organs. Living donors can also provide tissues capable of regeneration; these include blood, bone marrow, and the superficial layers of the skin. In the case of a vital, unpaired organ, such as the heart, the use of cadaver donors is obligatory. In practice, with the exception of blood and bone marrow, the great majority of transplanted organs are cadaveric in origin, a necessity that presents difficult logistic problems. The most serious problem restricting the use of allografts is immunological. Because cells in the donor graft express on their surface a number of genetically determined transplantation antigens that are not present in the recipient, allografts provoke a defensive reaction analogous to that evoked by pathogenic microorganisms. As a consequence, after a transient initial period of apparent well-being, graft function progressively deteriorates and the donor tissue is eventually destroyed.