This chapter provides a brief account of the development of studies on structure, composition, properties, cytopathology, and taxonomy of Ilarviruses. Ilarviruses frequently, though not always, have woody natural hosts. Their experimental host ranges are wide and include many herbaceous species. There are some unconfirmed reports of experimental vector-transmission of Ilarviruses. Thus tobacco streak virus is reportedly vectored by the thrips Frankliniella and prunus necrotic ringspot virus by a mite and a nematode. The Ilarvirus group was formerly divided into subgroups A and B based on the idea that some but not all members of each subgroup were serologically distantly related, whereas they were unrelated to members of the other subgroup. Classical embedding and thin sectioning, it is clear, are inadequate for studying Ilarvirus cytopathology, and newer techniques such as enzyme cytochemistry and immune labeling must be tried before useful results are obtained.