Many infertile couples cannot achieve pregnancy after repeatedly implemented assisted reproductive technology (ART) programs, as well as its successful prolongation, even with the transfer of good-quality euploid embryos. In this connection, repeated implantation failures (RIFs) in the in vitro fertilization (IVF) protocols and recurrent miscarriage (RM) are important medical and social problems. The generally accepted methods of examination do not explain the cause of repeated reproductive failures in 30-50% of cases. Among the causes of idiopathic reproductive losses, the immunological factor, including both systemic and local changes in the immune status, plays a leading role. Innate immune cells have been established to play a central role in the processes of implantation, placentation, formation, and maintenance of immune tolerance towards a developing embryo. Of great scientific and practical interest are natural killer (NK) cells that are the largest population of endometrial lymphocytes, changes in the quantitative and qualitative profile of which in both peripheral blood and the endometrium are associated with conditions, such as RIF, RM, uterine fibroids, endometriosis, preeclampsia and others. Conclusion. The conducted studies indicate the important role of both subpopulations of NK cells in a woman’s reproductive health. However, to date, there are insufficient data on the function and mechanisms of action of NK cells in reproductive losses, which requires further investigation.