Regulatory T cells emerge in the last years as key players in allowing fetal survival within the maternal uterus. They were shown to be a unique subpopulation of T cells expanding during human and murine pregnancy. The importance of Treg for a normal pregnancy situation was proven by studies showing that their absence impairs murine pregnancy while the adoptive transfer of Treg prevents fetal rejection. In humans, pregnancy pathologies are associated with lower Treg frequencies while therapies that improve pregnancy outcome are able to boost their number. Functional studies have shown that Treg can regulate immune cell responses directly at the fetal-maternal interface either by interacting with other cells or by inducing the expression of immune regulatory molecules. This article revises relevant literature on regulatory T cells in human and murine pregnancy.