OX40, also termed CD134 and TNFRSF4, is a T cell co-stimulatory molecule of the TNF receptor superfamily which plays a key role in the survival and homeostasis of effector and memory T cells. OX40 is expressed on CD4+ and CD8+ T cells upon engagement of the TCR by antigen presenting cells along with co-stimulation by CD40-CD40 Ligand and CD28-B7. The interaction between OX40 and OX40 ligand (OX40L) will occur when activated T cells bind to professional antigen-presenting cells (APCs). The T-cell functions, including cytokine production, expansion, and survival, are then enhanced by the OX40 costimulatory signals. OX40 signals are critical for controlling the function and differentiation of Foxp3+ regulatory T cells. OX40-OX40L interaction regulates T-cell tolerance, peripheral T-cell homeostasis, and T-cell-mediated inflammatory diseases.