The International Lymphoma Study Group has proposed aconsensus classification for lymphoid neoplasms. Lymphoidneoplasms are defined as distinct biological entities, based on acombination of morphologic, immunophenotypic, genetic, andclinical features. Each distinct disease may have a range ofhistologic grade and clinical aggressiveness. Although manydistinct diseases can now be recognized, three of them (follicular lymphoma, diffuse large B-cell lymphoma, and Hodgkin'sdisease) account for the majority of the cases seen in Europeand the USA. Recognition of distinct disease entities is essentialin order to develop and test effective therapies.