Louis-Hector Berlioz (11 December 1803 – 8 March 1869) was a French Romantic composer and conductor. His output includes orchestral works such as the Symphonie fantastique and Harold in Italy, choral pieces including the Requiem and L'Enfance du Christ, his three operas Benvenuto Cellini, Les Troyens and Béatrice et Bénédict, and works of hybrid genres such as the "dramatic symphony" Roméo et Juliette and the "dramatic legend" La Damnation de Faust.

The elder son of a provincial physician, Berlioz was expected to follow his father into medicine, and he attended a Parisian medical college before defying his family by taking up music as a profession. His independence of mind and refusal to follow traditional rules and formulas put him at odds with the conservative musical establishment of Paris. He briefly moderated his style sufficiently to win France's premier music prize – the Prix de Rome – in 1830, but he learned little from the academics of the Paris Conservatoire. Opinion was divided for many years between those who thought him an original genius and those who viewed his music as lacking in form and coherence.

At the age of twenty-four Berlioz fell in love with the Irish Shakespearean actress Harriet Smithson, and he pursued her obsessively until she finally accepted him seven years later. Their marriage was happy at first but eventually foundered. Harriet inspired his first major success, the Symphonie fantastique, in which an idealised depiction of her occurs throughout.

Berlioz completed three operas, the first of which, Benvenuto Cellini, was an outright failure. The second, the epic Les Troyens (The Trojans), was so large in scale that it was never staged in its entirety during his lifetime. His last opera, Béatrice et Bénédict – based on Shakespeare's comedy Much Ado About Nothing – was a success at its premiere but did not enter the regular operatic repertoire. Meeting only occasional success in France as a composer, Berlioz increasingly turned to conducting, in which he gained an international reputation. He was highly regarded in Germany, Britain and Russia both as a composer and as a conductor. To supplement his earnings he wrote musical journalism throughout much of his career; some of it has been preserved in book form, including his Treatise on Instrumentation (1844), which was influential in the 19th and 20th centuries. Berlioz died in Paris at the age of 65.

Berlioz was born on 11 December 1803, the eldest child of Louis Berlioz [fr] (1776–1848), a physician, and his wife, Marie-Antoinette Joséphine, née Marmion (1784–1838). His birthplace was the family home in the commune of La Côte-Saint-André in the département of Isère, in south-eastern France. His parents had five more children, three of whom died in infancy; their surviving daughters, Nanci and Adèle, remained close to Berlioz throughout their lives.

Berlioz's father, a respected local figure, was a progressively minded doctor credited as the first European to practise and write about acupuncture. He was an agnostic with a liberal outlook; his wife was a strict Roman Catholic of less flexible views. After briefly attending a local school when he was about ten, Berlioz was educated at home by his father. He recalled in his Mémoires that he enjoyed geography, especially books about travel, to which his mind would sometimes wander when he was supposed to be studying Latin; the classics nonetheless made an impression on him, and he was moved to tears by Virgil's account of the tragedy of Dido and Aeneas. Later he studied philosophy, rhetoric, and – because his father planned a medical career for him – anatomy.

Music did not feature prominently in the young Berlioz's education. His father gave him basic instruction on the flageolet, and he later took flute and guitar lessons with local teachers. He never studied the piano, and throughout his life played haltingly at best. He later contended that this was an advantage because it "saved me from the tyranny of keyboard habits, so dangerous to thought, and from the lure of conventional harmonies".

At the age of twelve Berlioz fell in love for the first time. The object of his affections was an eighteen-year-old neighbour, Estelle Dubœuf. He was teased for what was seen as a boyish infatuation, but something of his early passion for Estelle endured all his life. He poured some of his unrequited feelings into his early attempts at composition. Trying to master harmony, he read Rameau's Traité de l'harmonie, which proved incomprehensible to a novice, but Charles-Simon Catel's simpler treatise on the subject made it clearer to him. He wrote several chamber works as a youth, subsequently destroying the manuscripts, but one theme that remained in his mind reappeared later as the A-flat second subject of the overture to Les Francs-juges.