Members of the Bunyaviridae family are amongst the most widespread viruses in the world. They can be found on every inhabited continent at virtually every latitude, and are able to infect a wide range of arthropods, plants and mammals including humans. More than 300 named viruses are contained within the family Bunyaviridae (Virus Taxonomy: Seventh Report of the International Committee on Taxonomy of Viruses (2000) 599), and several members cause significant disease in humans or domestic animals. Despite being recognised as an emerging threat, relatively little is known about their virulence mechanisms. Here, we try to summarise the current state of knowledge about how the viruses of the Bunyaviridae succeed in establishing infection in the face of a powerful immune system.