Fidelio, originally titled Leonore, oder Der Triumph der ehelichen Liebe, Op. 72, is the only opera by German composer Ludwig van Beethoven. The libretto was originally prepared by Joseph Sonnleithner from the French of Jean-Nicolas Bouilly. The opera premiered at Vienna's Theater an der Wien on 20 November 1805. The following year, Beethoven's friend Stephan von Breuning rewrote the libretto, shortening the work from three acts to two. After further work on the libretto by Georg Friedrich Treitschke, a final version was performed at the Kärntnertortheater on 23 May 1814. As these libretto revisions were going on, Beethoven was also revising some of the music. By convention, only the final version is called Fidelio, and the others are referred to as Leonore.

Ludwig van Beethoven was a German composer and pianist. He is one of the most revered figures in the history of Western music; his works rank among the most performed of the classical music repertoire and span the transition from the Classical period to the Romantic era in classical music. His early period, during which he forged his craft, is typically considered to have lasted until 1802. From 1802 to around 1812, his middle period showed an individual development from the styles of Joseph Haydn and Wolfgang Amadeus Mozart, and is sometimes characterized as heroic. During this time, Beethoven began to grow increasingly deaf. In his late period, from 1812 to 1827, he extended his innovations in musical form and expression.