His first attempt, for the 1805 premiere, is believed to have been the overture now known as "Leonore No. 2". Beethoven then focussed this version for the performances of 1806, creating "Leonore No. 3". The latter is considered by many listeners as the greatest of the four overtures, but as an intensely dramatic, full-scale symphonic movement it had the effect of overwhelming the (rather light) initial scenes of the opera.
Beethoven accordingly experimented with cutting it back somewhat, for a planned 1808 performance in Prague; this is believed to be the version now called "Leonore No. 1", which is being performed tonight. Finally, for the 1814 revival Beethoven began anew, and with fresh musical material wrote what we now know as the Fidelio overture. As this somewhat lighter overture seems to work best of the four as a start to the opera, Beethoven's final intentions are generally respected in contemporary productions.