Graft-versus-host disease (GVHD) continues to be a major complication after allogeneic bone marrow transplantation, especially with the increasing use of unrelated and mismatched donors. Recently there has been renewed scientific interest in GVHD because of the increasing appreciation of the complexity of the immune responses seen in GVHD. Two basic aspects of the immune response in GVHD, the immunologic target and the effector mechanisms, are now more completely understood. First, the target of the immune response in GVHD has long been felt to be histocompatibility antigens possessed by the host, but not the donor. Recently, recognition of self antigens in GVHD has been documented, showing that GVHD is more complex than simple alloreactivity. Second, the effector mechanism in GVHD was initially felt to be direct cytotoxicity by alloreactive T cells. It is now recognized that cytokines play a central role in mediating many of the clinical and experimental manifestations of GVHD. The development in these two areas will be reviewed and the implications for clinical transplantation discussed.