The existence of a population of lymphocytes with suppressor activity able to inhibit immune responses has been widely studied. The greatest advances were made when researchers proposed markers, such as CD25 and Foxp3, for identifying those suppressor T cells. Regulatory T cells (Tregs) represent approximately 10% of the CD4 T cells and are able to suppress the immune responses to self and foreign antigens. Tregs can develop and acquire a suppressor phenotype in the thymus (natural Tregs) or be induced in the periphery followed certain activation and antigen presentations conditions (induced Tregs). These cells can suppress the immunological system by two principal pathways: the direct suppression of the target cells by cell-contact, and by the secretion of suppressor cytokines. This review summarizes the published data on Tregs and its suggested role in various states of health and disease.