The term satellite virus was adopted by Kassanis 1 to describe the small 17-nm diameter particle found in some but not all cultures of tobacco necrosis virus (TNV). This satellite virus depends on TNV for its multiplication, but is unrelated to it serologically and is not needed by TNV for its multiplication. The scope of the term satellite was later broadened to include nucleic acid species that behaved like satellite viruses but did not produce specific coat proteins and were found in particles that have the coat protein of the helper virus. 2 Satellites are therefore a diverse collection of particles and nucleic acid species that depend for their multiplication on one of the wide range of plant viruses. Table 1 lists known satellites as well as some RNA species that are very likely to be satellites; 26 viruses from more than 6 virus groups have satellites and several have more than one satellite. As more than half of these satellites have been described only in the last 4 years it seems likely that the list will continue to grow in coming years.