This chapter discusses general perspectives on the expression of RNA virus infections from cloned cDNA, the use of such expressible clones as a genetic tool for molecular studies of RNA viruses, and some of the broad conclusions arising to date from these studies. It focuses on in vivo analyses of positive strand RNA viruses in natural host cells and, in particular, on the example of brome mosaic virus, which is under study in the authors' laboratory. The genetic approach to virus function is the construction of recombinants between wild-type virus and phenotypic mutants or strains. A significant basic lesson from work to date is that the replication machinery of some RNA viruses will tolerate dramatic, arbitrary changes in the viral genome. Cloned viral cDNAs have been used to generate infections by two approaches: directly by transfection of the cloned cDNA itself, and indirectly by inoculation of transcripts synthesized in vitro from the cloned cDNA.