The first successful demonstration that effective T cell depletion can enable immune reconstitution without causing graft versus host disease (GVHD) was achieved in 1980 using lectin-separated hematopoietic stem cells. In leukemia patients undergoing supralethal radio- and chemotherapy, T cell-depleted transplants are vigorously rejected by residual host T cells; this barrier was first overcome in 1993 by the use of megadose stem cell transplants. This clinical observation can be explained, in part, by the demonstration that cells within the CD34 compartments, as well as their immediate early myeloid progeny, are endowed with veto activity. Engraftment of mismatched hematopoietic stem cells following reduced intensity conditioning, still represents a major challenge. Progress has been made recently by using anti-3rd party veto CTLs and T regulatory cells.