Many studies that took place after discovery of regulatory T (Treg) cells have widened our, still incomplete, knowledge about that specialized cell population. Treg cells differentiate mainly in the thymus, but this process occurs also in the periphery. They suppress proliferation of other cells (CD4, CD8, B lymphocytes, NK) in direct, cell to cell, interactions in vitro and therefore are main controllers of the proper immune response. Deficiency as well as functional dysregulation of this population may be responsible for development of autoimmune events.