Natural killer (NK) cell neoplasm is a heterogeneous disease group. In the latest World Health Organization (WHO) classification of tumours of hematopoietic and lymphoid tissues (2008), disease entities considered as NK-cell derivation include NK-lymphoblastic leukemia/lymphoma, chronic lymphoproliferative disorders of NK cells, aggressive NK-cell leukemia, and extranodal NK-cell lymphoma, nasal-type. Despite recent advances in NK-cell research, which have expanded our understanding of the biology of NK-cell neoplasm, it cannot yet be sharply delineated from myeloid neoplasms and T-cell neoplasms even in some "well-known" entity, such as extranodal NK/T-cell lymphoma. This review describes current knowledge of the biology of NK cells and pathology of NK neoplasms as classified in the 2008 WHO classification of tumours of hematopoietic and lymphoid tissues.