NK cells, or natural killer cells belong to the group of innate lymphoid cells which are vital to the innate immune system. They are a type of cytotoxic lymphocytes. NK cells respond rapidly to virus infected cell and other intracellular pathogens. They also take action against the formation of tumors. There are different NK cell receptors which have different functions. They can directly cause cell death after binding themselves to Fas ligand which shows infection of a cell. The MHC-independent receptors use a different pathway to cause apoptosis in infected cells. The activation of NK cells depends on the balance of activating and inhibitory receptor stimulation. This book unfolds the innovative aspects of NK cell receptors, which will be crucial for the progress of this field in the future. It consists of contributions made by international experts. The book is appropriate for students seeking detailed information in this area as well as for experts.