Anaplastic large cell lymphoma (ALCL) is a subtype of peripheral T-cell lymphoma (PTCL) first described in 1985 as a lymphoid malignancy characterized by marked cellular pleomorphism, propensity to grow cohesively, tendency to invade lymph node sinuses and diffuse expression of CD30 1. The discovery of the t(2;5), involving the anaplastic lymphoma kinase (ALK) gene on chromosome 2 and the nucleophosmin (NPM) gene on chromosome 5 in the majority of systemic ALCL, has soon pointed out that ALCL is a clinically and biologically heterogeneous disease. While ALK-positive (ALK+) ALCL is usually characterized by onset in children and young adults and better prognosis, epidemiology, poor outcome and possibly genetic defects of ALK-negative (ALK−) ALCL suggest that this neoplasms should be considered an independent pathological entity. The aim of this review is to illustrate clinical features, histology, immunophenotype, genetics and biology of ALCL and discuss possible relationship(s) among different T-non-Hodgkin lymphoma (T-NHL). Copyright © 2009 John Wiley & Sons, Ltd.