OBJECTIVE This review summarizes recent progress that has advanced our understanding of the pathobiology and prognosis of peripheral T-cell lymphomas including the changes introduced to the updated World Health Organization classification of lymphoid neoplasms. RESULTS The International Peripheral T-Cell Lymphoma Project was a large collaborative project, highlighting the clinico-pathologic and geographic differences of this diverse group of diseases. Peripheral T-cell lymphoma, not otherwise specified, is a biologically heterogeneous subtype, and a number of studies including investigations using molecular profiling have explored whether meaningful prognostic or biologic subgroups can be identified. Angioimmunoblastic T-cell lymphoma demonstrates overlapping immunophenotypical and molecular features with normal follicular T-helper cells, giving some insight into the cell of origin. Systemic anaplastic large cell lymphoma is composed of two disease groups based on the presence or absence of anaplastic lymphoma kinase overexpression. Anaplastic lymphoma kinase-positive anaplastic large cell lymphoma has a significantly better prognosis than anaplastic lymphoma kinase-negative anaplastic large cell lymphoma and molecular studies have illustrated that each entity has a distinct molecular profile. Anaplastic lymphoma kinase-negative anaplastic large cell lymphoma has been recently shown to have a more favourable, but still unsatisfactory, prognosis than peripheral T-cell lymphoma, not otherwise specified, and biological and molecular distinctions confirm that they are separate entities. CONCLUSIONS Advances are being made in understanding the unique pathobiology of the peripheral T-cell lymphoma subtypes that will help to refine diagnoses, identify potential prognostic markers, elucidate the molecular mechanisms critical in disease pathogenesis and define new therapeutic targets in the poor-risk population.