Pierre Louis Joseph Boulez (French: [pjɛʁ lwi ʒozεf bulɛz]; 26 March 1925 – 5 January 2016) was a French composer, conductor and writer, and the founder of several musical institutions. He was one of the dominant figures of post-war contemporary classical music.

Born in Montbrison, in the Loire department of France, the son of an engineer, Boulez studied at the Conservatoire de Paris with Olivier Messiaen, and privately with Andrée Vaurabourg and René Leibowitz. He began his professional career in the late 1940s as music director of the Renaud-Barrault theatre company in Paris. He was a leading figure in avant-garde music, playing an important role in the development of integral serialism in the 1950s, controlled chance music in the 1960s and the electronic transformation of instrumental music in real time from the 1970s onwards. His tendency to revise earlier compositions meant that his body of work was relatively small, but it included pieces considered landmarks of twentieth-century music, such as Le Marteau sans maître, Pli selon pli and Répons. His uncompromising commitment to modernism and the trenchant, polemical tone in which he expressed his views on music led some to criticise him as a dogmatist.

Boulez was also one of the most prominent conductors of his generation. In a career lasting more than sixty years, he was music director of the New York Philharmonic, chief conductor of the BBC Symphony Orchestra and principal guest conductor of the Chicago Symphony Orchestra and the Cleveland Orchestra. He made frequent appearances with many other orchestras, including the Vienna Philharmonic and the Berlin Philharmonic. He was known for his performances of the music of the first half of the twentieth century—including Debussy and Ravel, Stravinsky and Bartók, and the Second Viennese School—as well as that of his contemporaries, such as Ligeti, Berio and Carter. His work in the opera house included the production of Wagner's Ring cycle for the centenary of the Bayreuth Festival, and the world premiere of the three-act version of Berg's opera Lulu. His recorded legacy is extensive.

He also founded several musical institutions. In Paris he set up the Domaine musical in the 1950s to promote new music; in the 1970s he established the Institut de Recherche et Coordination Acoustique / Musique (IRCAM), to foster research and innovation in music, and the Ensemble intercontemporain, a chamber orchestra specialising in contemporary music. Later he co-founded the Cité de la musique, a concert hall, museum and library dedicated to music in the Parc de la Villette in Paris and, in Switzerland, the Lucerne Festival Academy, an international orchestra of young musicians, with which he gave first performances of many new works.

Pierre Boulez was born on 26 March 1925, in Montbrison, a small town in the Loire department of east-central France, to Léon and Marcelle (née Calabre) Boulez. He was the third of four children: an older sister, Jeanne (1922–2018) and younger brother, Roger (b. 1936) were preceded by another child called Pierre (b. 1920), who died after a few months. Léon (1891–1969), an engineer and technical director of a steel factory, is described by biographers as an authoritarian figure with a strong sense of fairness, and Marcelle (1897–1985) as an outgoing, good-humoured woman, who deferred to her husband's strict Catholic beliefs, while not necessarily sharing them. The family prospered, moving in 1929 from the apartment above a pharmacy, where Boulez was born, to a comfortable detached house, where he spent most of his childhood.

From the age of seven Boulez went to school at the Institut Victor de Laprade, a Catholic seminary where the thirteen-hour school day was filled with study and prayer. By the age of eighteen he had repudiated Catholicism; later in life he described himself as an agnostic.

As a child, Boulez took piano lessons, played chamber music with local amateurs and sang in the school choir. After completing the first part of his baccalaureate a year early, he spent the academic year of 1940–41 at the Pensionnat St. Louis, a boarding school in nearby Saint-Étienne. The following year he took classes in advanced mathematics at the Cours Sogno in Lyon (a school established by the Lazaristes) with a view to gaining admission to the École Polytechnique in Paris. His father hoped this would lead to a career in engineering. Wartime conditions in Lyon were already harsh; they became harsher still when the Vichy government fell, the Germans took over and the city became a centre of the resistance.

In Lyon, Boulez first heard an orchestra, saw his first operas (Mussorgsky's Boris Godunov and Wagner's Die Meistersinger von Nürnberg) and met the soprano Ninon Vallin, who asked him to play for her. Impressed by his ability, she persuaded his father to allow him to apply to the Conservatoire de Lyon. He was rejected but was determined to pursue a career in music. The following year, with his sister's support in the face of opposition from his father, he studied piano and harmony privately with Lionel de Pachmann (son of the pianist Vladimir de Pachmann). "Our parents were strong, but finally we were stronger than they", Boulez later said. In the event, when he moved to Paris in the autumn of 1943, hoping to enroll at the Conservatoire de Paris, his father accompanied him, helped him to find a room (in the 7th arrondissement) and subsidised him until he could earn a living.