The publication of this volume of The Viruses entitled The Togaviridae and Flaviviridae comes at an appropriate time. The structure and rep- lication strategies of these viruses are now known to be sufficiently di- verse to warrant the removal of flaviviruses from the Togaviridae family and establish them as an independent family. Flaviviridae have a special place in the history of virology. The prototype virus-yellow fever virus- was the first virus to be identified as the cause of a human disease. Some of the history of this discovery is described in Chapter 1 of this volume; in Chapter 10 the complete sequence of the RNA genome of the virus is presented. This sequence not only defines the primary structure of the viral proteins, it also clarifies the mechanism of translation of the fla- vivirus genome. Knowledge of the sequence of the structural proteins of these viruses represents an important step in the potential goal of using purified flavivirus glycoproteins as vaccines. Many of the chapters in this volume focus on the structure and replication of the Togaviridae. These viruses have provided valuable models for studies in cell biology, partic- ularly with regard to the cotranslational and posttranslational steps re- quired for the synthesis and localization of membrane glycoproteins. Fur- thermore, Togaviridae have been pivotal in our growing understanding of how enveloped viruses enter and exit from cells. The broad outlines of the structure and gene expression of Togavir- idae and Flaviviridae are known, but important questions remain.