Peripheral T-cell lymphomas (PTCLs) are a rare and heterogeneous group of lymphoproliferative disorders of postthymic origin. Progress in elucidating the pathobiology and appropriate therapy of these neoplasms has been slow, primarily because of their rarity, but also because until the early 1990s, they were generally grouped together and combined with B-cell lymphomas. It is now understood that most PTCLs are highly aggressive and respond poorly to standard chemo therapy, and thus they have a significantly poorer prognosis than their B-cell counterparts. In 1994, the Revised European and American Lymphoma classification provided the first uniform system to classify lymphoproliferative disorders on the basis of morphology, phenotype, genetics, and clinical features. Since then, the World Health Organization (WHO) has refined this system as additional information has evolved to further elucidate the origin of these neoplasms. More recently, several new distinct and provisional categories in the updated WHO classification have been defined. This review summarizes the changes in the fourth edition of the WHO classification of lymphoid tumors, with particular focus on the aggressive subtypes.