During gestation, CD4+/CD25+ regulatory T cells (TREG) play crucial role in mechanisms by which maternal immunological system tolerates semiallogeneic fetus. Paternal antigens as well as maternal hormons contribute to the pregnancy- associated increase of TREG population. TREG regulate immunological response by cell-cell contact and indirectly by cytokines. TREG population expands during human pregnancy, that may be detected both in peripheral blood and in decidua. At the fetal-maternal interface TREG create tolerant microenviroment by interaction with other immune cells and by inducing the expression of immune regulatory molecules such as TGF-b, LIF or HO-1. The importance of TREG for a normal pregnancy was proven by many studies. Diminished numbers of CD4+/CD25+ regulatory T cells or their improper function is associated with implantation failure, fetal rejection, miscarriage and pre-eclampsia.