The mammalian target of rapamycin (mTOR) is an evolutionarily conserved serine-threonine kinase that is known to sense the environmental and cellular nutrition status to control cell growth. In immunity, mTOR is essential for both the proper activation and subsequent proliferation of effector T cells, yet also restrains the development of regulatory T cells. However, in monocytes/macrophages and peripheral myeloid dendritic cells, mTOR restricts proinflammatory and promotes anti-inflammatory responses, whereas, in plasmacytoid dendritic cells, mTOR fosters type I interferon production. These results place mTOR in a novel immunoregulatory context that highlights the potential of mTOR inhibitors as both immunosuppressant and anti-cancer agents.