To conduct meaningful scientific discussion, the parlies involved need to agree about the subject being discussed. Making this agreement is taxonomy. Thus, defining taxa, classifying lower laxa or individuals into higher taxa. and naming them are fundamental to scientific discussion and progress. This need for taxonomy has been recognized in virology, implicitly or explicitly, for many years, In this lime, iherc have been many and various propositions as to how lo classify viruses of plants. These have largely been attempts to make order of virus diversity by using as much knowledge of the diversity as was available at the time. Inevitably, any new information prompted revision of these schemes, often in radical ways. This process has been reviewed in Matthews (1983). Current taxonomic systems for viruses date from the years immediately preceding the establishment in 1966 of the International Committee on Nomenclature of Viruses (ICNV). A major background influence on this initiative was the then newly acquired knowledge about particle morphology. These observations led lo proposals for a variety of classification schemes, in particular those of Brandes and Wetter (1959) for plant viruses, and that of Lwol'f el al. (1 962) for all viruses, based on various characlcrs in a highly structured hierarchical scheme.