The pathogenesis of acute graft versus host disease (GVHD) is multistep process. This review considers acute GVHD in three sequential steps: conditioning regimen, donor T cell activation, and effector mechanisms. In step one, the conditioning regimen simultaneously damages and activates host tissues, amplifying antigen presentation to allogeneic donor T cells. In step two, donor T cells, activated by host alloantigens, proliferate and secrete a variety of cytokines. Type 1 cytokines (interleukin-2 and interferon-y) are critical for acute GVHD, but several regulatory mechanisms of tissue damage include inflammatory cytokines and cytolytic cellular effectors. The gastrointestinal (GI) tract is a principal target organ because damage to the GI mucosa can release inflammatory mediators such as endotoxin that amplify systemic disease. The inflammatory processes of acute GVHD can be considered as a distortion of the cellular responses to viral and bacterial infections. Cell-mediated toxicity is critical to other GVHD target organs, particularly the liver, where Fas-mediated injury predominates. The cytolytic pathways (e.g., perforin) clearly intensify acute GVHD, although they are not necessary for systemic disease in several model systems. Many of these insights come from animal models using mutant mouse strains that can clarify the role of individual proteins or cell types in the disease process. These insights should allow the testing of new classes of drugs and inhibitors in clinical bone marrow transplantation.