Oil price crisis: What is Opec? In 60 seconds
The global price of oil slumped after the Opec oil producers' cartel decided not to cut output at its meeting in Vienna.
Opec's secretary general Abdallah Salem el-Badri said the 12-member The Organization of the Petroleum Exporting Countries would not try to shore up prices by reducing production.
Following the announcement Brent crude fell below $72 a barrel, hitting lows previously seen in August 2010.
BBC News assesses the group, its history and the challenges ahead - in 60 seconds.
Video produced by Michael Hirst