The nine chapters presented in this book provide contemporary reviews of research on defective RNAs, satellite RNA viruses, and dependent RNA viruses that require the presence of a helper virus in order to establish productive infections. Since their initial identification nearly four decades ago, fundamental analyses of pathogenic and interdependent interactions involving these agents have contributed enormously to our appreciation of virus structure, RNA replication, and processes leading to disease. Findings arising from these studies have also advanced numerous ancillary areas, including...