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Natural killer (NK) cells are important effector cells of innate immune system implicated in many physiological processes including elimination of cancer cells and virus infected cells. NK cells comprise a majority of large granular lymphocytes circulating in peripheral blood with a minority derived from T cell lineage. Even though NK cells were first described more than 40 years ago, it was not until the 1980s and 1990s when immunophenotyping was incorporated into clinical diagnostic methods and resulted in discovery of distinct disorders of large granular lymphocytes and NK cells. Since then, significant progress was made in our better understanding of immunophenotypic and genotypic characteristics, biology, functions as well as disorders of these cells. Most recently, clinical studies using NK-cell based immunotherapy have shown promising results in treatment of some of malignant diseases. Disorders of large granular lymphocytes and NK cells are rare comprising only about 1% of all lymphoid malignancies in western countries. The rarity of these conditions was a main reason that the progress in our understanding of pathogenesis and development of novel therapeutic approaches has been delayed compared to development in more common B cell lymphoid malignancies. The low incidence of these diseases and scarcity of prospective clinical trials also limit the availability of evidence based research literature as well as comprehensive reviews about NK cell disorders. Thus, the editors decided to take on the challenging task and summarize our current knowledge about malignant and benign diseases of large granular lymphocytes in this book based on the best available evidence. The editors selected topics most relevant to clinical practice in order to provide a useful guide for practicing physicians. Chapters describing four disorders (T-cell large granular lymphocytic leukemia, chronic lymphoproliferative disorder of NK cells, extranodal NK cell lymphoma and aggressive NK cell leukemia) incorporated into most recent 2016 revision of classification of lymphoid malignancies are separated into experimental, diagnositc and clinical parts for easier understanding and reading. We are aware of challenges and inherited limitations of any larger project like this one due to a rapid progress especially in the field of genomics, which may not be incorporated in this book before it is published. The editors and contributing authors would like to thank the publisher NOVA for their support.