The paper shows the characteristics of important immune cells, which are natural killer (NK) cells. Those are the cells which carry different roles in immunity, as well in physiology and diseases. Natural killer cells have a huge cytotoxical potential and are able to “kill” other types of cells. Moreover, they have cytotoxic granules and secrete many cytokines and chemokines, what improves immunity caused by NK cells. An interesting data concerning the division of NK cells’ subsets have been provided, as those data evaluated over last couple of years. Nowadays, the division is made on the basis of phenotypic, functional and anatomical properties, and such subpopulations can be enumerated: CD56^dim CD16+, CD56^brightCD16–, CD56+/NKp46+ and NKT (natural killer T) cells and among uterine NK (uNK) cells, endometrial NK (eNK) and decidual NK (dNK) cells. Taking into account the fact that the population of NK cells is growing, their functions are also wider. There are several roles that have been studied in this paper, such as anti-infection and anti-cancer immunity, as well as their pivotal role during pregnancy.