During the past decade, there have been many advances in our understanding of Hodgkin's disease. Among the most important is the discovery that the Reed-Sternberg cell is a lymphoid cell, in most cases a B cell, and that it is clonal. Hodgkin's disease is thus a true lymphoma, deserving of a name change to Hodgkin's lymphoma (HL). On the basis of a combination of immunophenotype and morphologic features, the Revised European-American classification system recognizes two main types of HL: classical types (nodular sclerosis, mixed cellularity, lymphocyte-rich classical HL, and lymphocyte depletion) and nodular lymphocyte-predominant type. These two types probably are distinct biologic entities. The immunophenotype and genetic features of the classical HL and the nodular lymphocyte-predominant type have been defined. These are useful in the subclassification of HL and in distinguishing HL from two recently described, aggressive lymphomas that were in the past often diagnosed as HL, i.e., anaplastic large-cell lymphoma, T-cell type, and T-cell/histiocyte-rich large B-cell lymphoma. Epstein-Barr virus has been detected in approximately 40% of the cases of classical HL, and it is clonal; this suggests that this virus might play a role in the pathogenesis of at least some types of HL. Alternatively, its presence might simply reflect the prevalence of Epstein-Barr virus-infected B cells in the individual. Despite the advances of the past 10 years, many questions remain to be answered; these will provide the challenges of the next decade.