**1. Introduction**

Natural killer (NK) cells represent about 5–15% of circulating lymphocytes [1] and belong to the innate immune system, in particular due to their invariant antigen receptors. Morphologically, as opposed to "small lymphocytes," most of them are typically large azurophilic granules containing lymphocytes (LGL) [2], characterized by high cytoplasmic:nuclear ratio. They have initially been referred to as k or null lymphocytes (non-B and non-T cells), with the highest cytotoxic capacity [3, 4], because they lack conventional T-cell membrane markers and the surface membrane immunoglobulin (smIg) [5, 6]. NK cells mediate resistance against intracellular pathogens, mainly viral-infected, bacteria-infected and protozoa-infected cells, and have the potential to restrain cancer and metastasis. They are also able to contribute to the activation and the regulation of the immune responses, as well as the orientation of adaptive immune responses [7, 8]. More recently, NK cells were recognized as a subtype of type 1 innate lymphoid cells (ILC1), which express the transcription factor T-box expressed in T cells (T-bet), and defined by the production of the T helper cell type 1 (Th1)-associated cytokine interferon gamma (IFN-γ) and the inability to produce Th2 cell-associated and Th17 cell-associated cytokines [9]. As a result of collective work, this book has therefore the ultimate purpose to address the most fundamental aspects of NK cells, as well as their clinical applications for cancer immunotherapy.
