Tumor necrosis factor receptor (TNFR) family members such as glucocorticoid-induced TNFR (GITR) control T cell activation, differentiation, and effector functions. Importantly, GITR functions as a pivotal regulator of physiologic and pathologic immune responses by abrogating the suppressive effects of T regulatory cells and costimulating T effector cells. However, the molecular mechanisms underlying GITR-triggered signal transduction pathways remain unclear. Interestingly, GITR-induced stimulation of TNFR-associated factor (TRAF) 5-deficient T cells resulted in decreased activation of nuclear factor kappaB as well as the mitogen-activated protein kinases p38 and extracellular signal-regulated protein kinase, whereas activation of c-Jun N-terminal kinase was less affected. Consistent with impaired signaling, costimulatory effects of GITR were diminished in TRAF5-/- T cells. In sum, our studies indicate that TRAF5 plays a crucial role in GITR-induced signaling pathways that augment T cell activation.