In this chapter, we would like to assess the possible link between endometriosis and spontaneous/recurrent miscarriage or implantation failure and to focus on potential immunological explanations for such an association. This chapter we reviewed the lack of association between endometriosis (and its treatment), spontaneous abortion and recurrent abortion, and the possible association between endometriosis and reproductive failure after IVF. Although several immunological factors have been identified that could affect/impair embryo implantation and spontaneous fertility in patients with endometriosis, it appears that the impaired reproductive outcome after ART in women with endometriosis when compared to controls is related to alterations within the follicle or oocyte, resulting in embryos with a decreased ability to implant or to lead to a live birth.