The recently updated World Health Organization (WHO) classification of tumors of 
hematopoietic and lymphoid tissues, published in 2008, has made great advances 
in revising the disorders previously included in the pool of natural killer (NK) 
cell tumors. Although NK cell neoplasms represent a relatively rare group of 
diseases, accounting for <5% of all lymphoid neoplasms, they include very 
distinctive conditions both clinically and pathologically. This family of 
diseases includes the most indolent clinical forms, such as the provisional new 
entry of chronic lymphoproliferative disorder of NK cells (CLPD-NK) in the WHO 
classification, as well as one of the most fatal diseases recognized in medical 
oncology, aggressive NK cell leukemia (ANKL), which is characterized by a 
prognosis of weeks, or even days. In addition, some disorders previously 
identified as blastic NK cell lymphoma within the NK cell system have been more 
properly defined and included in the blastic plasmacytoid dentritic cell 
neoplasms, although rare cases of bona fide immature NK lymphoid tumors (now 
classified as NK cell lymphoblastic leukemia/lymphoma) have been reported in the 
literature. This paper focuses on recent concepts and progress in morphology, 
pathogenesis, clinicopathological features, treatment approaches, and outcomes 
of NK cell malignancies.