The mature (postthymocyte; peripheral) T/natural killer (NK) lymphomas/leukemias represent 5–15% of all non-Hodgkin lymphoma (NHL) and vary according to geography. Peripheral T cell lymphoma, not otherwise specified (PTCL, NOS), is the most common type worldwide. There are more nodal presentations in Europe and North America, where the second most common types in each region are angioimmunoblastic T cell lymphoma (AITL) and anaplastic large cell lymphoma (ALCL), respectively. There is more extranodal disease in Asia, due to Epstein–Barr virus related NK/T lymphoma and human T cell leukemia virus (HTLV)-1 associated adult T cell leukemia/lymphoma (ATLL). With the exceptions of the indolent mycosis fungoides (MF) and the chemo-sensitive anaplastic lymphoma kinase (ALK)-positive ALCL, the prognosis in most peripheral T/NK neoplasms is poor, with a 5-year survival less than 30%. (Table 25.1)