The Togaviridae are a family of 4 genera of enveloped RNA viruses with icosahedral nucleocapsids: Alphaviruses, Flaviviruses, Rubiviruses and Pestiviruses (Melnick, 1980). The Alpha- and Flaviviruses are arthropod-borne viruses which can replicate in both vertebrate and arthropod cells. Both Alpha- and Flaviviruses are endemic in bird and animal populations and normally infect humans only as an incidental host (Shope, 1980). In humans, Alphaviruses cause fever, encephalitis, arthritis and rashes as well as inapparent infections. TheFlaviviruses also cause fever and encephalitis as well as some hemorrhagic fevers. Rubi- and Pestiviruses are not arthropod-borne and apparently replicate only in vertebrate cells (Shope, 1980). The Rubiviruses are represented by rubella virus, which causes a mild febrile exanthem with teratologic effects in humans. Pestiviruses are responsible for hog cholera, bovine diarrhea and border disease of sheep, important diseases of domestic animals. Among the Togaviridae, the replication and maturation of Alpha- and Flaviviruses have been studied most extensively and will be considered separately in this chapter.