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. 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 pNK cell phenotypes. CD56dim CD16+ NK cells represent 90% of the pNK cell population and have cytotoxic tendencies, while CD56bright CD16- cells are less cytotoxic but produce the majority of cytokines important for immunological regulation and represent 10%. The presence of receptors on the surface of NK cells contributes to their function (Table 1). Uterine natural killer cells: uNK cells, also called decidual NK (dNK) cells, are present in the decidua, which is the lining of the uterus. . This mucosa is in close contact with fetal trophoblast cells (Fig. 1) and is necessary to separate the fetus from the maternal circulatory system. uNK cells have the CD56bright CD16- phenotype and are therefore similar to the less cytotoxic pNK cells. Research by Spornitz (1992) showed that uNK cells have different stages of growth (Fig. 2) and their numbers decrease significantly after the first trimester. The origin of uNK cells has been debated and different theories have been suggested. Yamaguchi et al. (2006) suggest...... middle of article ......d decidua', Advances in Anatomy, Embryology and Cell Biology, 124, pp. Tripathi, P., Naik, S. and Agrawal, S. (2007) “Role of HLA-G, HLA-E and KIR2DL4 during 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, HOD (2013) “Uterine NK cells regulate endometrial bleeding in women and are suppressed by the progesterone receptor modulator asoprisnil”, The Journal of Immunology, 191, pp. 000-000.22. Yamaguchi, T., Kitaya, K., Daikoku, N., Yasuo, T., Fushiki, S., Honjo, H. (2006) “Potential L-selectin ligands involved in the selective recruitment of the natural killer CD16 (- ) peripheral blood cells in the human endometrium”, Reproductive Biology, 74, pp. 35-40.