Hematopoietic stem cells have been shown to provide the best chance for long term patient engraftment in bone marrow transplantation, peripheral blood stem cell transplantation, and umbilical cord blood transplantation. Characteristics of these cells include the ability to divide without differentiating; presence in bone marrow, peripheral blood, and cord blood; and negativity for all lineage CD markers. Autologous, syngeneic, and allogeneic transplantations have all been performed and patients respond well. Complications include graft rejection or failure and graft-versus-host disease; however, such complications can be minimized and treated. Hematopoietic stem cell transplantation holds promise for the treatment of many malignancies and metabolic diseases that have not responded to other forms of treatment.