The endometrium consists of epithelium of the surface with endometrial glands, which are surrounded by stroma. During pregnancy, the uterine mucosa cells are structurally and functionally transformed into decidual cells. The decidua produces various cytokines, growth factors and hormones in order to enable trophoblast adhesion and invasion. A successful implantation depends on decidualization as well as the development of a certain immune tolerance to fetal tissue. This immune tolerance is inter alia achieved by expression of specific surface molecules and secretion of immunosuppressive molecules. Failures during decidualization result in more or less severe pregnancy complications and in the worst case in an abortion. A malformation which commonly leads to implantation failures is endometriosis. Within this paper, the functions of the decidua and subsequently the implantation failures that go with it, as well as the immunological changes during pregnancy will be depicted.