Regulatory T (T_reg) cells constitute a dynamic population that is essential for controlling immune responses in health and disease. Defects in T_reg cell function and decreases in T_reg cell numbers have been observed in patients with autoimmunity and the opposite effects on T_reg cells occur in cancer settings. Current research on new therapies for these diseases is focused on modulating T_reg cell function to increase or decrease suppressive activity in autoimmunity and cancer, respectively. In this regard, several co-inhibitory receptors that are preferentially expressed by T_reg cells under homeostatic conditions have recently been shown to control T_reg cell function and stability in different disease settings. These receptors could be amenable to therapeutic targeting aimed at modulating T_reg cell function and plasticity. This Review summarizes recent data regarding the role of co-inhibitory molecules in the control of T_reg cell function and stability, with a focus on their roles and potential therapeutic use in autoimmunity and cancer.