Forkhead box P3 (FOXP3)⁺ regulatory T (T_Reg) cells prevent autoimmune disease, maintain immune homeostasis and modulate immune responses during infection. To accomplish these tasks, T_Reg cell activity is precisely controlled, and this requires T_Reg cells to alter their migratory, functional and homeostatic properties in response to specific cues in the immune environment. We review progress in understanding the diversity of T_Reg cells, T_Reg cell function in different anatomical and inflammatory settings, and the influence of the immune environment on T_Reg cell activity. We also consider how these factors affect immune-mediated disease in the contexts of infection, autoimmunity, cancer and transplantation.