To prepare for the presence of a fetus, the uterus undergoes a critical process termed decidualisation that, in mice and probably humans, is required for reproductive success. This process involves phenotypic changes in uterine stromal cells that enable them to support implantation and placental development. Accompanying this transformation is the recruitment of a specialised population of maternal immune cells. These cells, termed decidual leukocytes, are a significant presence, composing up to 70% of cells in the uterus throughout early pregnancy. At first glance, the recruitment of immune cells to the site of implantation seems at odds with maternal tolerance. However, the unique characteristics of decidual leukocytes, particularly with regard to their abundance and distinct composition, suggest that they participate in protecting fetal tissues from immune attack. Thus, the mechanisms that recruit decidual leukocytes to the uterine wall provide an important component to successful pregnancy.