Natural killer (NK) cells are able to kill tumour cells or virus-infected cells spontaneously and independent of classical MHC-restriction. The activities of these cells can be regulated by different cytokines. Interleukin-2 has been shown to be a potent stimulatory factor for NK cells, inducing proliferation, cytokine production and increased cytotoxic activity. During recent years, the influence of various cytokines on NK cells have been studied, like interferons, IL-1, IL-6, IL-4 and TGF-β. Recently, new cytokines like IL-7 and IL-12 have been shown to possess direct and potent stimulatory effects on NK cells. Furthermore, endogenous production of TNF seems to play an important role in mediating effects induced by several stimulatory cytokines. Here, an overview of the different regulatory effects of these cytokines on NK cells are presented.