Book excerpt: Japanese encephalitis and West Nile viruses are members of the Japanese encephalitis serological group of the genus Flavivirus and therefore closely related genetically and antigenically. They share a number of properties, including the use of birds as their major wildlife maintenance host and Culicine mosquitoes for transmission, and they are both associated with severe human disease, as well as fatal infections in horses. The emergence of these two viruses, and their well-established propensity to colonise new areas, make it timely to re-examine their ecology, biology, molecular structure, replication and epidemiology, and these therefore provide the focus of this volume.