Natural killer cells (NK cells) are subpopulation of innate lymphoid cells with unique and significant role in control of tumor growth and metastasis. NK cells are constantly attracting interest in research, despite the fact that their initial knowledge is about the ability to lyse the tumor cells without major histocompatibility complex (MHC) restriction. NK cells were immunophenotypically characterized as CD3−/CD56+ /CD16+/Nkp60+ cells. It is believed that their activation is largely determined by the balance between specific activation and inhibitory receptors expressed on their surface, unlike T cells. After activation, they have the ability to lyse tumor cells by a secretory mechanism, by production of several cytokines, granzymes, and perforin, as well as by other nonsecretory mechanism mediated by death receptor–induced apoptosis in target cells. NK cell dysfunction has been shown during cancer progression. Many cytokines exogenously have the ability to potentiate NK activity while many others, mostly pro-inflammatory, lead to their inhibition. However, the possibilities of cell therapy and the principles of immunomodulation that have recently been intensified in the research of these cells give a great hope for the treatment of patients with tumors.