Boris Godunov (, Borís Godunóv) is an opera by Modest Mussorgsky (1839–1881). The work was composed between 1868 and 1873 in Saint Petersburg, Russia. It is Mussorgsky's only completed opera and is considered his masterpiece. Its subjects are the Russian ruler Boris Godunov, who reigned as Tsar (1598 to 1605) during the Time of Troubles, and his nemesis, the False Dmitriy (reigned 1605 to 1606). The Russian-language libretto was written by the composer, and is based on the drama Boris Godunov by Aleksandr Pushkin, and, in the Revised Version of 1872, on Nikolay Karamzin's History of the Russian State.