Acute Graft-versus-host disease (GVHD) is a major complication after allogeneic hematopoietic stem cell transplantation. Although this process is thought to consist of several phases, T-cell activation plays a critical role in the pathogenesis of acute GVHD. To become efficient effectors, T-cells require additional costimulation after T-cell receptor signaling. A number of molecules are involved in costimulation of T-cells such as CD28, CD40L, CD30, OX40, 4-1BB, ICOS, and LIGHT. The system is regulated by inhibitory molecules, CTLA-4, and PD-1. There is experimental evidence that those molecules are implicated in the pathogenesis of GHVD. We describe how these molecules are involved in acute GVHD and how the blockade of costimulatory molecules may have potential implications for the treatment of patients with acute GVHD.