The capped and polyadenylated genomes of coronaviruses, spanning some 27 to 31 kb, are the largest of all RNA virus genomes, including those of the segmented RNA viruses. This chapter presents the reverse genetics of the largest RNA viruses. Just as all other positive-sense RNA viruses (retroviruses excluded), coronavirus genomic RNA is infectious when transfected into the cells of a permissive host. Therefore, in principle, the most direct way to perform reverse genetics on a coronavirus ought to involve the construction of a full-length genomic complementary DNA (cDNA) clone from which infectious RNA could be transcribed in vitro. The method––targeted recombination––is less direct and more laborious, and so far it has been applied exclusively to site-directed mutagenesis of mouse hepatitis virus (MHV). Thus, at least for structural gene mutations that are not expected to be severely deleterious, targeted recombination may remain the less complicated alternative for the creation of MHV mutants. The chapter discusses targeted RNA recombination, such as development of system, genetic analysis of coronavirus structural proteins, genetic analysis of coronavirus RNA synthesis, and limitations of targeted recombination.