Rift Valley fever (RVF) is an arthropod-borne viral disease of ruminants, camels and humans. It is also a significant zoonosis which may be encountered as an uncomplicated influenza-like illness, but may also present as a haemorrhagic disease with liver involvement; there may also be ocular or neurological lesions. In animals, RVF may be inapparent in non-pregnant adults, but outbreaks are characterised by the onset of abortions and high neonatal mortality. Jaundice hepatitis and death are seen in older animals. Outbreaks of RVF are associated with persistent heavy rainfall with sustained flooding and the appearance of large numbers of mosquitoes, the main vector. Localised heavy rainfall is seldom sufficient to create conditions for an outbreak; the simultaneous emergence of large numbers of first generation transovarially infected mosquitoes is also required. After virus amplification in vertebrates, mosquitoes act as secondary vectors to sustain the epidemic.