In 2008, the World Health Organization (WHO) published a revised and updated edition of the classification of tumors of the hematopoietic and lymphoid tissues. The aims of the fourth edition of the WHO classification was to incorporate new scientific and clinical information in order to refine diagnostic criteria for previously described neoplasms and to introduce newly recognized disease entities. The recognition that T-cell lymphomas are related to the innate and adaptive immune system, as well as enhanced understanding of other T-cell subsets, such as the regulatory T-cell and follicular helper T-cells, has contributed to our understanding of the morphologic, histologic, and immunophenotypic features of T- and NK-cell neoplasms. The purpose of this review is to highlight major changes in the classification of T- and NK-cell neoplasms and to explain the rationale for these changes.