Tissue lymphocytes keep our body healthy every day. Special NK cells in the uterus populate the mucosa at every menstrual cycle and become more numerous during pregnancy. Uterine NK cells engage in a molecular conversation with the placental precursor cells (fetal trophoblast) and this dialogue helps the local vasculature to accommodate the needs of the growing fetus, influencing pregnancy outcomes. About 140 million babies are born every year and it is estimated that ~10 million, as well as their mothers, may be affected by adverse outcomes (e.g., pre-eclampsia and fetal growth restriction). We have not yet worked out the mechanistic details of these developmental processes, which are not only key for the health of the mother and her fetus, but also for the long-term health of the offspring. Here we discuss recent advances, what we know and what we don’t know about uterine NK cells, including their established functions in vascular remodeling and control of trophoblast invasion, their potential functions in implantation and memory of previous pregnancies, and their acquired immune functions in anti-microbial responses. We also discuss recent technical advances and highlight outstanding questions. Only when we figure out the precise functions of uterine NK cells and the tissue microenvironment at the maternal-fetal interface, can we start to implement modern immunotherapy approaches to improve pregnancy outcomes and the health of the next generation.