Natural killer cells are a type of cytotoxic lymphocytes which are a part of the innate immune system. These cells respond actively to virus infected cells, intracellular pathogens and tumor formation. Major histocompatibility complex (MHC) is presented on the infected cells surfaces. These MHCs are detected by the immune cells and cytokine release is triggered causing lysis or apoptosis of the infected cell. Natural killer cells allow a much faster immune reaction as they can recognize and kill stressed cells even in the absence of antibodies and MHC. They differentiate and mature in the lymph nodes, bone marrow, tonsils, spleen and thymus. Then, they enter into the circulation. The objective of this book is to give a general view of the role of natural killer cells in health and disease. It presents researches and studies performed by experts across the globe. The extensive content of this book provides the readers with a thorough understanding of the subject.