Natural killer (NK) cells have been at the forefront of immunology for two decades. During that time, a great amount of information about these cells has been obtained. They are important in antiinfectious and antitumoral defense and shape the adaptive immune response. In addition, they can act as immunoregulatory cells. In recent years, the therapeutic potential of NK cells in cancer immunotherapy has become increasingly evident. This book describes in detail current knowledge about NK cells and covers a broad range of NK cell-related topics, including those that are not frequently reviewed, e.g. NK cells and allergy or NK cells and skin diseases.