West Nile virus (WNV) is an RNA virus and a member of the Flaviviridae family. The recent geographical expansion of WNV into areas where no activity had been previously reported has been highlighted by the detection of WNV in North America. There is also a recent trend for more numerous and serious outbreaks in Eurasia. The main hosts are birds and the principle vectors are mosquitoes, usually of the genus Culex. Although most infected people do not become symptomatic, severe diseases such as encephalitis and, less commonly, aseptic meningitis may occur, more frequently in the elderly. The public can be protected by giving advice on the avoidance of mosquito bites and by the implementation of ecological surveillance and measures to reduce the mosquito population. While a few human cases have been identified in returning travellers, WNV has not been reported in any animal or bird in the UK. However, this may simply indicate that the diagnosis has not been sought. Potential avian hosts and mosquito vectors of WNV are present in the UK and birds migrate to the UK from areas of endemic WNV activity. However, the population density of mosquitoes is relatively low and therefore the risk of WNV being transmitted in the UK is thought to be low. We lack sufficient information on the ecology of the virus, and on mosquito populations, to accurately determine this risk. Clinicians are advised to consider WNV as a differential diagnosis, especially in patients over 50 years old with a clinical picture of viral encephalitis or aseptic meningitis presenting in the summer months.