Objectives: Natural killer (NK) cells are part of innate immunity and play an important role in early immunological defense against different types of infections as well as tumors. On their cell surface they express a range of activating and inhibitory receptors through which they can eliminate potentially dangerous cells while sparing healthy ones. It is believed that NK cell activation is achieved by shifting the balance of signals, received through activating and inhibitory receptors, towards activation. However, recently it has been questioned whether this system is sensitive enough, and if there is an additional requirement for a “master signal” to reach full NK cell activity. Our hypothesis is that CD16, NKG2D and NCR1 are these “master regulators”, whose triggering is essential for NK cells to gain full activation, and our objective was to investigate the dependence of NK cell activation on these three receptors. Materials and methods: Triple knock out (TKO) mice lacking CD16, NKG2D and NCR1 receptors were generated. Phenotypic analysis of NK cells was done in the spleen, bone marrow, kidney, liver and blood of mice. To determine the impact of triple deficiency on the functionality of NK cells in vitro, proliferation assay as well as stimulations with different cytokines and through different receptors, were done. Using various tumor targets, killing capacity as well as cytokine production of TKO NK cells was analyzed in co-cultivation assay. Next, mice were inoculated with RMA-S lymphoma and B16 melanoma to test the functionality of TKO NK cells in vivo. Furthermore, B16 tumor infiltrating NK cells (TANKs) were analyzed. To investigate how TKO mice control viral infections a model of murine cytomegalovirus (mCMV) was used. Finally, to confirm that the findings were NK cell intrinsic, we generated and analyzed mixed bone marrow chimeras. Results: Phenotypic analysis revealed a more mature phenotype of NK cells in the periphery, whereas there were no changes in the number and frequency of NK cells. Alternations in the receptor expression as well as hyper-responsiveness through NK1.1 receptor were present. In vitro killing of RMA-S, B16 and YAC-1 tumor targets was comparable to the wild type controls, while in vivo TKO mice had a significant defect in the control of B16 melanoma. Analysis of B16 TANKs revealed that TKO NK cells were less activated and produced lower amounts of Perforin and IFNγ. The latter is known to be an important cytokine for the control of B16 melanoma. mCMV infection resulted in higher viral titers in the lungs and spleen, indicating a compromised control of mCMV. Conclusion: Although the results of this work show that CD16, NKG2D and NCR1 receptors are not the “master regulators” of NK cell activity, the analysis of TKO mice revealed incredible compensatory mechanisms of NK cells. The lack of three important activating receptors: CD16, NKG2D and NCR1 results in worse control of viral infection, as well as certain tumors, but this deficiency is mostly compensated, so the development and activation of NK cells is largely comparable to that of control animals. The observed compensation is a result of change in the level of expression as well as the activity of individual receptors on NK cells. The obtained knowledge provides a deeper insight into the biology of NK cells, indicating their incredible plasticity and ability to compensate, which can potentially be useful in the further development of NK cell dependent immunotherapie