Tristan und Isolde, WWV 90, is an opera in three acts by Richard Wagner to a German libretto by the composer, based largely on the 12th-century romance Tristan and Iseult by Gottfried von Strassburg. It was composed between 1857 and 1859 and premiered at the Königliches Hoftheater und Nationaltheater in Munich on 10 June 1865 with Hans von Bülow conducting. Wagner referred to the work not as an opera, but called it "eine Handlung".

Wilhelm Richard Wagner was a German composer, theatre director, polemicist, and conductor who is chiefly known for his operas. Unlike most opera composers, Wagner wrote both the libretto and the music for each of his stage works. Initially establishing his reputation as a composer of works in the romantic vein of Carl Maria von Weber and Giacomo Meyerbeer, Wagner revolutionised opera through his concept of the Gesamtkunstwerk, by which he sought to synthesise the poetic, visual, musical and dramatic arts, with music subsidiary to drama. He described this vision in a series of essays published between 1849 and 1852. Wagner realised these ideas most fully in the first half of the four-opera cycle Der Ring des Nibelungen.