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 structure and function of nucleic acids and proteins, nucleoprotein interactions, translational mechan- isms, RNA processing, macromolecular evolution, and a plethora of other specialty topics. Research on these subviral pathogens is continuing to illuminate various aspects of biology, chemistry, and biotechnology, so the book is intended to provide a current treatment that will be useful for readers with interests in topics related to these areas. Observations in the early I 960s first revealed that a defective virus, satellite tobacco necrosis virus (STNV), is associated with and depends on the presence of a hel per virus, tobacco necrosis virus (TNV), for its multiplication. This finding ushered in a new era in virology that soon resulted in a more profound apprecia- tion of the relationships of viruses and their interactions with each other.