Forkhead box P3 (FOXP3)-expressing regulatory T (T_Reg) cells have a pivotal role in the regulation of immune responses and in the maintenance of immunological self-tolerance. These cells have emerged as attractive targets for strategies that allow the steering of immune responses in desired directions — arming the immune system to destroy infected cells and cancer cells or downregulating it to limit tissue destruction in autoimmunity. Efforts to understand the generation, activation and function of T_Reg cells should permit the development of therapeutics for reprogramming the immune system. In this Review, we discuss insights into the generation of T_Reg cells, their involvement in disease and the molecular basis of the dominant tolerance exerted by FOXP3⁺ T_Reg cells that could permit their safe and specific manipulation in humans.