Since the publication of a comprehensive review on West Nile virus (WNV) in 2002, there has been substantial progress in understanding of transmission, epidemiology, and geographic distribution of the virus and manifestations of disease produced by the infection. There have also been advances in development of diagnostic and therapeutic agents and vaccines. Nevertheless, many questions about the epidemic remain unanswered, and several new issues have arisen-for example: whether the epidemic will increase as the virus spreads to the Pacific coast of North America; whether arthropods other than mosquitoes will act as vectors for the infection; whether WNV will spread to South America and cause an epidemic there; whether the distribution of WNV in Asia and Europe will increase; and whether adaptation of WNV to new ecosystems will produce viruses with altered genetic and phenotypic properties. This review aims to provide an update on knowledge of WNV biology that can be used to highlight the advances in the field during the past 2 years and help to define the questions that academic, industrial, and public-health communities must address in development of measures to control WNV disease.