Retinoic acid (RA) is produced by a number of cell types, including macrophages and dendritic cells, which express retinal dehydrogenases that convert vitamin A to its main biologically active metabolite, all-trans RA. All-trans RA binds to its nuclear retinoic acid receptors that are expressed in lymphoid cells and act as transcription factors to regulate cell homing and differentiation. RA production by CD103+ dendritic cells and alveolar macrophages functions with TGF-β to promote conversion of naive T cells into Foxp3+ regulatory T cells and, thereby, maintain mucosal tolerance. Furthermore, RA inhibits the differentiation of naive T cells into Th17 cells. However, Th1 and Th17 responses are constrained during vitamin A deficiency and in nuclear RA receptor α–defective mice. Furthermore, RA promotes effector T cell responses during infection or autoimmune diseases. Thus, RA plays a role in immune homeostasis in the steady-state but activates pathogenic T cells in conditions of inflammation.