Maternal tolerance against fetal antigens is still one of the unsolved questions in pregnancy. Focusing on the various subsets of immune cells playing in concert with the human immune system, tolerance induction is nowadays often accredited to a specialized group of immune cells, the antigen-presenting cells (APC). There are surprisingly few reports about APC populations in the decidualized endometrium, the decidua where fetal cells get into contact with the maternal immune system. Nowadays it seems to be clear that at least three populations of APC, the macrophages, dendritic cells and immature, monocyte-derived APC, could be found in the decidua of a pregnant uterus. This chapter summarizes the characteristics of dendritic cells and macrophages as APC in general and focuses on the description and characterization of APC in the decidua of different species found in the literature.