The adoptive transfer of donor-type CD4⁺CD25⁺FOXP3⁺ regulatory T cells (Treg) protects from graft-versus-host disease in murine bone marrow transplantation models. Results from first clinical trials exploring such strategies have recently been presented and seem to confirm the efficacy of Treg for the prevention of this severe complication after allogeneic stem cell transplantation. Further improvements in Treg isolation and in vitro expansion technologies will facilitate the broader exploration of Treg therapies, for example, for the treatment of ongoing graft-versus-host disease or the prevention of graft rejection after solid organ transplantation.