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CLASSIFICATION & NOMENCLATURE of VIRUSESA large number of morphologically and physico-chemically distinct types of viruses that infect virtually all classes of living organisms have been described. Generating some order from the chaos that existed with respect to the naming and cataloguing of viruses became the task of the International Committee on the Nomenclature of Viruses (ICNV), which first met in 1 966. In keeping with the spirit of taxonomy, the committee changed its name to the International Committee on the Taxonomy of Viruses in 1 973, which is what it is today. The 7thReport of the ICTV was published in Spring, 2000. It contained three orders, 56 families, 9 subfamilies, 233 genera, and 1 550 virus species. Descriptions of satellite viruses, viroids, prions, and retrotransposons are included.The concept of classifying viruses on the basis of virus properties as opposed to host affinities or biological effects has integrated the field and allowed for a sensible approach to the study of comparative virology. A centralized repository of virus information, the ICTV database (ICTVdb) is under construction and nearly completed. It includes information gathered from several large databases from around the world. The availability of such information has facilitated the job of yaccurate identification and diagnosis of new and important virus diseases. The characterization of viruses and their assignment to specific groups and families has had a major impact on what properties virologists must examine in order to properly identify and classify the causal agent of a specific virus disease. Our understanding of the interrelationships among different viruses, genera, and families continues to increase, much through comparative sequence analysis. We therefore continue to gain more confidence in the system we use to classify viruses. Until recently, many viruses were not associated with specific taxa and most plant viruses were categorized into groups rather than families or genera. As more sequence data have accumulated, more viruses have been placed in newly described or existing taxa, and the “group” designation has given way to categorization into families or genera of existing families. Some viruses are still are not associated with named families, but with the current taxonomic framework solidly in place, this number is being reduced with each report of the ICTV.j