Bone marrow transplantation (BMT) has the potential to treat hemoglobinopathies (sickle cell and thalassemia) autoimmunity (diabetes, lupus, multiple sclerosis, rheumatoid arthritis, Crohn's colitis) and enzyme deficiency states. Graft versus host disease (GVHD) is a major complication and limitation to the therapeutic application of BMT. There have been many clinical trials and experimental animal models that have attempted to control GVHD through the engineering of the donor bone marrow cells (BMC). Historically, several methods have demonstrated effectiveness in controlling GVHD; however they were also associated with a marked increase in the rate of graft failure. Highly purified hematopoietic stem cells (HSC) engraft quite readily in genetically-matched recipients while they do not engraft as easily in MHC-disparate recipients. The numbers of HSC must be increased 100-200 fold in order to overcome the allogeneic barrier. We were the first to phenotypically and to functionally characterize a novel cell in the bone marrow that enables engraftment of highly purified HSC in allogeneic recipients. The discovery of graft facilitating cell populations has resulted in the restoration of the engraftment-potential of purified HSC between genetically-disparate individuals. The addition of facilitating cells (FC) to T cell-depleted BMC grafts results in allogeneic engraftment without GVHD or graft failure. New strategies of BMC engineering that retain FC and HSC but avoid GVHD have allowed successful engraftment in mismatched and older recipients. These techniques have expanded the therapeutic potential of BMT to virtually every candidate as well as to non-malignant diseases in which the morbidity associated with conventional BMT could not be accepted. This article reviews the transition of the FC technology from bench to bedside and discuss the potentially broad-reaching applications of BMT and mixed chimerism.