Graft-versus-host disease (GVHD) is caused by immunologic recognition of the patient by the donor marrow graft after allogeneic bone marrow transplantation. Despite advances in understanding and new agents and methods to treat GVHD, the disease and complications related to it and its treatment are the principal causes of death after transplantation. This article reviews the pathogenesis, prediction, prophylaxis, and treatment of both acute and chronic GVHD.