Malignancies of natural killer (NK) cells have increasingly been recognized as distinct clinicopathological entities. The tumor cells are characterized by an immunophenotype of CD2+, surface CD3-, cytoplasmic CD3epsilon+, and CD56+. The T cell receptor gene is in germline configuration, and a consistent association with Epstein-Barr virus is demonstrable. Pathologically, the tumor cells show variable cytological appearances, with frequent angioinvasion and angiocentricity associated with zonal necrosis. Clinically, most cases affect the nasal cavity or other parts of the upper aerodigestive tract, and are referred to as nasal NK cell lymphoma. A minority involve extranasal sites such as the skin, gastrointestinal tract and testis, and are often referred to as extranasal NK cell lymphoma. A particularly aggressive form presents fulminantly as disseminated disease, sometimes with a leukemic phase, and is referred to as aggressive NK cell lymphoma/leukemia. Cytogenetic and molecular analysis have shown DNA losses at chromosomes 6q, 11q, 13q and 17p to be recurrent aberrations in NK cell malignancies. Frequent DNA gains are also found in chromosomes 1p, 6p, 11q, 12q, 17q, 19p, 20q, and Xp. These regions of DNA losses and gains should be targets for further investigation in order to understand the molecular pathogenesis of this lymphoma. Finally, optimal treatment modalities need to be determined, as all subtypes of NK cell malignancies are associated with a poor prognosis.