Recent studies have underscored the importance of regulatory T cells (Treg) in the maintenance of immunological self-tolerance and in the prevention of autoimmune diseases. Regulatory T cells is heterogenic subpopulation of T cells, that is able to suppress functions of effector cells during the immune response. Among them are natural (CD4+CD25+) and induced Treg (Tr1, Th3, CD4+CD25-) that gain their unique fenotype during the development in the thymus or in the periphery, respectively. CD25, CD45RO, CD152, GITR, LAG-3, several adhesion molecules, chemokine receptors as well as Toll-like receptors are detected on the surface of Treg. Mechanism of suppression used by natural Treg, although not completely understood, seems to depend more on the cell-cell contact than on cytokines (at least in vitro).