Considering the allograft rejection as a basic feature of the immune system, the mammalian pregnancy is an immunologic paradox where the semiallogeneic fetus is not rejected. How are the demands of pregnancy solved in the context of the maternal immunity? Medawar`s original proposal of maternal immune inertness during pregnancy should be revised to active materno-placental tolerance. Multiple mechanisms are involved in the peripheral and local tolerance induction that prevents fetal rejection while maintaining competent immune surveillance and protection. The goal of this review is to discuss the major cellular and molecular components of the immune system during pregnancy that control and promote fetal survival.