Natural Killer cells (NK cells) are cells of the innate immune system that are important players in mounting an immune response against viral infections and tumors. NK cells have a variety of mechanisms in order to detect and attack their target cells. In this review, the roles of NK cells in anti-tumor immunity have been discussed. NK cells use a combination of cell surface receptors and secreted factors in order to attack tumor cells and prevent tumor growth. In addition, NK cells act as an important mediator in the activation of the adaptive immune system, thereby resulting in a sustained anti-tumor immune response. Thus a high NK cell number in the tumor is indicative of a positive prognosis. Therefore, NK cell proportion is an important readout in order to evaluate the efficacy of novel anti-tumor therapies. However, a novel subset of NK cells .....