Pestiviruses infect a wide range of domestic, captive and free-living ruminants. Among domestic livestock, Border disease virus is a well recognised cause of an important congenital disease of sheep in virtually all sheep-rearing countries of the world. The clinical signs, pathogenesis, diagnosis, epidemiology and control of this disease are described in detail. One natural outbreak of Border disease in domestic goats has been described and there is serological and virological evidence that pestiviruses occur widely in this species. A pestivirus has been isolated from a farmed red deer (Cervus elaphus) and there is serological evidence of a widespread low prevalence of infection among this new domestic species. Pestiviruses have been associated also with outbreaks of disease among captive ruminants in zoological collections. Among free-living ruminants, pestiviruses have been recovered from dead roe deer (Capreolus capreolus), fallow deer (Dama dama), African buffalo (Syncerus caffer), giraffe (Giraffa camelopardalis) and wildebeest (Connochaetes spp.) but in all these instances the contribution of the virus to the cause of the disease was uncertain. Serological surveys have shown that many species of free-living ruminants in North America, Europe and Africa have varying prevalence rates of antibodies to pestiviruses.