In 1982 Stein and coworkers identified a new molecule, CD30 (Ki-1), which is expressed by Reed-Sternberg (RS) cells of Hodgkin's Disease (HD) (1). Although CD30 is not a specific RS cell marker, its characterization has assumed an important role not only in the differential diagnosis of HD, but also in the identification of a morphologically and clinically distinct type of large cell lymphoma, now designated as anaplastic large cell lymphoma (ALCL) (2). The cloning of human and murine CD30 and the utilization of genetically manipulated animal models have rapidly expanded our knowledge on its physiological role in lymphoid development and differentiation. The goal of this review is to present an overview of this rapidly evolving field and discuss the role of CD30 in normal and neoplastic lymphoid cells.