Introduction: Natural Killer cells play an important role in the development of the placenta during the early stages of pregnancy. The mechanism of angiogenesis is controlled in part by the effect of uterine natural killer (uNK) cells and their production of cytokines. The mechanisms and role of uNK cells will be examined, and the importance of uNK cells will become clear when disorders related to dysfunctional NK cells are examined. Peripheral natural killer cells: Natural killer (NK) cells are lymphocytes that are part of the innate immune system and have different functions depending on their location. Peripheral NK (pNK) cells recognize MHC class 1 molecules on the surface of stressed cells. There are two phenotypes of pNK cells. CD56dim CD16+ NK cells constitute 90% of the pNK cell population and have cytotoxic tendencies, while CD16- CD56bright cells are less cytotoxic but produce most of the cytokines important for immunological regulation and constitute 10%. The presence of receptors on the surface of NK cells contributes to their function (Table 1). Uterine Natural Killer Cells: uNK cells, also known as decidual NK (dNK) cells, are present in the decidua, which is the lining of the uterus. . This lining is in close contact with the fetal trophoblast cells (Fig. 1) and is necessary to separate the fetus from the maternal circulatory system. uNK cells have the CD56bright CD16- phenotype, so they are similar to less cytotoxic pNK cells. Research by Spornitz (1992) demonstrated that uNK cells have different stages of growth (Fig. 2) and that their number decreases dramatically after the first trimester. The origin of UNK cells has been debated and several theories have been suggested. Yamaguchi et al. (2006) suggest...... half of paper ......d decidua', Advances in Anatomy, Embryology and Cell Biology, 124, pp. 1-99.20. Tripathi, P., Naik, S. and Agrawal, S. (2007) 'Role of HLA-G, HLA-E and KIR2DL4 in pregnancy', International Journal of Human Genetics, 7(3), pp. 219-233.21. Wikens, J., Male, V., Ghazal, P., Forster, T., Gibson, DA, Williams, ARW, Brito-Mutunayagam, SL, Craigon, M., Lourenco, P., Cameron, IT, Chwalisz, K., Moffett, A. and Critchley, H.O.D. (2013) "Uterine NK cells regulate endometrial bleeding in women and are suppressed by the progesterone receptor modulator asoprinil", The Journal of Immunology, 191, pp. 000-000.22. Yamaguchi, T., Kitaya, K., Daikoku, N., Yasuo, T., Fushiki, S., Honjo, H. (2006) 'Potential sorting L ligands involved in selective recruitment of the natural killer CD16(-) peripheral blood cells in the human endometrium', Biology of Reproduction, 74, pp. 35-40.