described in 1985 as a neoplastic proliferation of lymphoid cells that are anaplastic in appearance (Figure 1A), have a propensity to grow cohesively, invade lymph node sinuses (Figure 1B) and consistently express the CD30 molecule. Major breakthroughs came with the discovery that some ALCL tumors carried the t(2;5) translocation which caused fusion of the nucleophosmin gene (NPM1) with a previously unrecognized gene, named anaplastic lymphoma kinase (ALK). Since then, a large bulk of information has accumulated on the role of ALK in the molecular pathogenesis of ALCL. Notable progress was also made in characterizing ALCL with the generation of specific monoclonal antibodies that permitted immunohistochemical detection of ALK chimeric proteins directly on fixed paraffinembedded biopsy samples. These reagents aided in clarifying controversies over the wide morphological spectrum of ALCL and in better defining the borders between ALCL and other lymphoma subtypes, such as Hodgkin’s lymphoma and peripheral T-cell lymphoma (PTCL). ALCL was first included as an entity in the Revised European and American Lymphoma (REAL) classification in 1994, and, subsequently, in the World Health Organization (WHO) classification of lymphoid neoplasms in 2001. The new edition of the WHO classification (2008) recognizes, within the spectrum of mature Tcell neoplasms, two types of systemic ALCL according to ALK protein expression in tumor samples: (i) a distinct entity, named ALK ALCL, which is characterized by ALK gene rearrangements and ALK protein expression; and (ii) a provisional entity, the so-called ALK ALCL, which cannot be distinguished morphologically from ALK ALCL but differs from this entity because of the lack of ALK protein.