Allogeneic hematopoietic cell transplantation provides effective control of hematopoietic malignancies, but with an associated risk of graft-versus-host disease (GVHD) related morbidity and mortality. Several advances in hematopoietic cell transplantation including high resolution HLA typing, development of reduced intensity conditioning regimens, infectious prophylaxis and treatment, and novel immunosuppressive agents have resulted in improved outcomes and improved access to transplantation, but GVHD remains a major obstacle. This clinico-pathological syndrome, mediated by donor alloreactive T cells, occurs often despite prophylactic immunosuppressive therapy. Regulatory T cells, a suppressive subset of the T-cell repertoire, may offer promise as a novel cellular therapy for more effective prevention of GVHD. While advances have been made in pre-clinical experimental animals, several challenges remain in the translation of this work to human trials. Strategies to effectively produce ex vivo expanded alloantigen-specific regulatory T cells specific for ubiquitous alloantigens but sparing hematopoietic- or tumor-associated antigens hold promise to prevent GVHD while allowing a preserved graft versus malignancy effect.