The term satellite virus was adopted by Kassanis 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. This chapter describes the properties of the different satellites together with the effects that the satellites have on their helper virus and on the symptoms that the helper virus induces in infected plants. It contains brief descriptions of the results of molecular studies, in particular, determination of nucleotide sequences on the structure and expression of satellite RNA. The nepovirus group contains many viruses which, among other properties, share that of having isometric particles, some of which are empty protein shells that lack RNA.