Abstract: The function of the immune system is to protect our system from pathogens entering our body and react immunologically to cells from a genetically different person. At the same time, the immune cells maintain tolerance to our own body cells and do not reject them. This is called as self-tolerance. However, in case of autoimmune diseases and cancer, the immune system identifies our cells as foreign and generates immune reactions resulting in severe damage to the affected organ. However, on the other hand, the cancer cells are eliminated or rejected by our immune system in the early stages but support their growth at later stage. These immune reactions controlling the self-tolerance are mediated by T regulatory cells which maintain immune homeostasis under healthy condition by keeping the effector CD4+ and CD8+ T cells under regulated state. Upon loss of immune balance T regulatory cells cannot suppress the effector T cells’ activity resulting in autoimmune or autoinflammatory reactions. This review will discuss the various subtypes of T regulatory cells and their role in autoimmune diseases and cancer.