Les Troyens is a French grand opera in five acts, running for about five hours, by Hector Berlioz. The libretto was written by Berlioz himself from Virgil's epic poem the Aeneid; the score was composed between 1856 and 1858. Les Troyens is Berlioz's most ambitious work, the summation of his entire artistic career, but he did not live to see it performed in its entirety. Under the title Les Troyens à Carthage, the last three acts were premièred with many cuts by Léon Carvalho's company, the Théâtre Lyrique, at their theatre on the Place du Châtelet in Paris on 4 November 1863, with 21 repeat performances. The reduced versions run for about three hours. After decades of neglect, today the opera is considered by some music critics as one of the finest ever written.

Grand opera is a genre of 19th-century opera generally in four or five acts, characterized by large-scale casts and orchestras. The original productions consisted of spectacular design and stage effects with plots normally based on or around dramatic historic events. The term is particularly applied to certain productions of the Paris Opéra from the late 1820s to around 1860; 'grand opéra' has sometimes been used to denote the Paris Opéra itself.