Bluetongue is a non-contagious disease of domestic and wild ruminants caused by a virus within the Orbivirus genus of the family Reoviridae and transmitted by Culicoides biting midges. It is a reportable disease of considerable socioeconomic concern and of major importance for the international trade of animals and animal products. In the past, bluetongue endemic areas were found between latitudes 40°N and 35°S; however, bluetongue has recently spread far beyond this traditional range. This is in accordance with the extension of areas in which the biting midge Culicoides imicola, the major vector of the virus in the “Old World”, is active. After 1998 new serotypes of bluetongue virus (BTV) were discovered in Southern European and Mediterranean countries. Since 2006 BTV-serotype 8 has also been reported from the countries in Northern and Western Europe where Culicoides imicola has not been found. In such cases, BTV is transmitted by Palearctic biting midges, such as C. obsoletus or C. dewulfi, and the disease has thus spread much further north than BTV has ever previously been detected. New BTV serotypes have recently been identified also in Israel, Australia and the USA. This review presents comprehensive information on this dangerous disease including its history, spread, routes of transmission and host range, as well as the causative agent and pathogenesis and diagnosis of the disease. It also deals with relevant preventive and control measures to be implemented in areas with bluetongue outbreaks.