Human Natural Killer (NK) cells are present in great number in pregnant uterine mucosa. They must be there for specialized functions, but which ones? This review discusses important recent observations that further contribute to this fascinating debate. Firstly, an array of corroborating findings indicates that uterine NK cell proliferation is synchronized with the cyclic surge of progesterone. Secondly, uterine NK cells are unlikely to exert a direct control on the embryo implantation. Thirdly, these NK cells influence the uterine vascular remodeling in early pregnancy but might not be the single key element that control trophoblast invasion. Finally, uterine NK cells are likely to be an important component of the local maternal immune response to pathogen infections.