This chapter discusses the comparison between the genomes of plant RNA viruses, such as, alfalfa mosaic virus, tobacco mosaic virus (TMV), and tobacco rattle virus (TRV). It presents data on the variability in the genomes of closely related virus strains and the variability that can be observed within a given virus strain. The chapter also discusses the possibility to select mutants from a given virus population and the stability of such mutants. Tobraviruses have been divided into three clusters containing strains of TRV, strains of pea early browning virus (PEBV), and Brazilian isolates represented by the CAM strain. Tobamoviruses have been classified into ten definite subgroups on the basis of the biochemical and immunological characteristics of their coat proteins. The polyprotein encoded by M-RNA of tomato black ring virus contains small homologies with the corresponding cowpea mosaic virus protein and with the TMV-Cc 30-kdalton protein.