The mature T/natural killer (NK) lymphoma/leukemias represent 5–15% of all non-Hodgkin lymphoma. These diseases have a geographic variation, with more nodal disease in North America and Europe, including peripheral T cell lymphomas, unspecified, anaplastic large cell lymphoma, and angioimmunoblastic T cell lymphoma; and more extranodal disease in Asia due to Epstein-Barr virus–related nasal NK/T lymphoma and human T-cell leukemia virus (HTLV)-1–associated adult T cell leukemia/lymphoma. The prognosis in most peripheral T/NK neoplasms is poor, with 5-year survival less than 30%. Progress has been slow due to the rarity of the diseases, geographic variation, relative chemoresistance, and lack of randomized trials. There is no consensus about optimal therapy in T/NK neoplasms, and recommendations are based on anecdotal reports, small series, and phase II trials. In this review, topics include the question of CHOP as standard therapy, prognostic factors, disease-adapted therapy, novel approaches, monoclonal antibody therapy, and stem cell transplantation.