This reference work is a product of the international VIDE (Virus Identification Data Exchange) project, which uses the DELTA (Description Language for Taxonomy) database system to collect diagnostic information on all plant viruses. Information on over 500 characters has been sought for each virus and stored in the VIDE database. Over two-thirds of these characters record the susceptibility, or otherwise, of a range of commonly used test plant species; the remainder represent the great range of other characters used for virus identification. The work covers more than 900 viruses, which represents the entire database of plant viruses from around the world. It includes all viruses of plants, not only those whose virions have been described, but also those such as umbraviruses that have no virion protein genes of their own, and use the virion proteins of their symbiotic helper viruses. Viroids and the many transmissible agents that cause virus-like diseases, but whose infectious particles have not yet been characterized, are excluded. However, the database accession numbers of the genomic sequences of satellite RNAs are included. The work also contains summaries of data for all the viruses that are definitive or tentative members of the 50 or so accepted genera or groups .