The leading French composer of the nineteenth century, and a great conductor, music writer, and orchestrator. He was successor to Beethoven as high priest of the Romantic style. All his works are passionate and personal, though in his own lifetime they were not appreciated enough.

His single best-known work is the Symphonie fantastique of 1830, when he had fallen in love with the Irish actor Harriet Smithson, having seen her as Ophelia and thus also discovered one of his other great loves, Shakespeare; and when he had finally won the Prix de Rome, the prize by which young composers were allowed to live and work in Rome.

His greatest opera is the epic Les Troyens, based on the Aeneid; others are Benvenuto Cellini and Beatrice and Benedict. His standalone overtures, such as Les Francs-juges, Roman Carnival, Rob Roy, Waverley, and King Lear are also very popular.

He wrote a Requiem, a Te Deum, choral works L'Enfance du Christ and La Damnation de Faust, and the extraordinary Grande symphonie funèbre et triomphale (to commemorate the July Revolution). These show his characteristic full use of lots of brass, and plentiful work for choirs.

Other works include the viola concerto Harold in Italy, based on Byron, and written for Paganini, who had given Berlioz a generous sum to free him to compose, but who however at first rejected the concerto as too difficult. Another symphony with vocals was Romeo and Juliet; Lélio was a sequel to the Symphonie fantastique; and he wrote a song cycle called Nuits d'été.

He expanded the orchestra that Beethoven used, and the modern assemblage is sometimes known as the Berlioz orchestra. I'm no musician so for technical details I'll quote a to-me incomprehensible passage from Grove:

Though Berlioz's compositional style has long been considered idiosyncratic, it can be seen to rely on an abundance of both technique and inspiration. Typical are expansive melodies of irregular phrase length, sometimes with a slight chromatic inflection, and expressive though not tonally adventurous harmonies. Freely contrapuntal textures predominate, used to a variety of fine effects including superimposition of separate themes; a striking boldness in rhythmic articulation gives the music much of its vitality.
Louis-Hector Berlioz was born in Côte-Saint-André in the Isère on 11 December 1803. His father destined him for medicine, which he studied for a short time, but he resisted his parents and preferred his musical studies. He went to the Conservatoire in 1826 and entered for the Prix de Rome four times. After winning it he returned to Paris in 1832.

Although he had fallen in love with Harriet Smithson seeing her on stage, it was only now that he met her. They married in October 1833: it was not alas a happy marriage, despite the romantic beginning; they separated in 1842 and she died, mentally unbalanced, in 1854. This enabled him to marry his partner, the singer Marie Recio. She died in 1862, his and Harriet's son Louis died in 1867, and Berlioz died in Paris on 8 March 1869.

Much of his life and his attitudes to music are documented in his eminently readable Memoirs. He also wrote a treatise on instrumentation. Because his music was not popular, a lot of his income came from journalism, and he was a champion of a number of other great musicians. He travelled widely in his conducting duties, from England to Hungary to Russia. He was enthusiastic for literature: the influence of Shakespeare, Virgil, Byron, and Scott is very evident.