Essentially a rebadging of the Conservatives' controversial city technology colleges, academies are state-funded, but privately- sponsored independent secondary schools that operate outside many of the normal regulations that state schools must abide by.
The scheme got off to a relatively slow start, with only three academies opening in the first tranche in September 2002.
But since then it has become a flagship in the drive by Blairites to introduce more choice into public services. The Government wants 200 academies open or in development in England by the end of 2010.
Charles Clarke, former education secretary, claimed academies could have a "bazooka effect" on educational achievement in their areas and exam results from the first ones have been promising.
But the scheme has also created controversy. Concerns have been expressed about the huge influence sponsors can have over academies in return for only a small percentage of thepound;23 million building costs and none of the running costs.
The TES revealed in August that two academies had paid out large sums of money to companies in which their private- sector sponsors had interests.
Teachers in academies do not have to be registered with the General Teaching Council. As independent state schools, even all-age academies will not have to abide by class-size rules.
Teaching unions fear academies will draw away pupils and money for building works from other schools. But they have largely proved popular with parents.