Peripheral T cell lymphoma (PTCL) is a term that encompasses a wide range of histological subtypes of Non-Hodgkin's lymphoma which arise from mature or post-thymic T lymphocytes. These tumours account for 10–20% of all cases of non-Hodgkin's lymphoma in the Western World. With the exception of HTLV1 associated lymphoma, there is little evidence that the incidence of PTCL is increasing. However, the diagnosis of PTCL is being made with greater frequency due to both an increased awareness of the entity and the more widespread use of phenotypic and genotypic studies for characterising lymphoid populations. The clinical course of PTCL is variable and less well documented than that for B cell lymphomas. As with B cell lymphomas, stage of disease is a major determinant of prognosis although the importance of histological subtype is still uncertain. Aggressive combination chemotherapy may result in complete remission and perhaps long term survival, but further prospective studies are urgently needed.