Allogeneic hematopoietic cell transplantation combines the power of cytotoxic chemo/radiotherapy with the ability of the new immune system to seek out and destroy tumor cells. However, administration of such myeloablative transplants is fraught with risks, some of which are related to the intensive conditioning regimens. Reductions in the intensity of the administered cytotoxic therapy have demonstrated that under some circumstances, the burden of fighting tumor and enhancing stem cell engraftment can be shouldered mostly by the transplanted immune system. Reduced intensity has allowed a potentially curative therapy for hematologic malignancies to be offered to an expanded patient population. Ongoing research seeks to enhance the safety and power of this form of allogeneic immunotherapy.