Pierre Boulez in 2011. (Christophe Ena/AP)

Pierre Boulez, who began his career as a radical modernist composer dedicated to overthrowing classical traditions and lived to become one of the most revered and sought-after orchestral conductors in the world, died Jan. 6 in Baden-Baden, Germany. He was 90.

His family announced the death to the Philharmonie Paris but did not disclose a cause.

Mr. Boulez (pronounced boo-LEZZ) came to public attention as the leading voice of postwar avant-garde music in France, an enfant terrible given to making public suggestions such as “the most elegant way of solving the opera problem would be to blow up the opera houses.”

Not only did he outlive that youthful fervor, he went on to conduct Richard Wagner’s “Ring” Cycle at Bayreuth in Germany, that most exalted — and most conservative — of operatic shrines, and enjoyed long and fruitful artistic relationships with many of the world’s great orchestras.

Throughout his life, however, he remained devoted to new and unusual music. During his bracing tenure as music director of the New York Philharmonic from 1971 to 1978, he led the orchestra through more contemporary works than it had played since the 19th century.

Pierre Boulez in 2008 (Rolf Haid/AFP/Getty Images)

He spoke of conducting in visual terms. “I conceive of a composition as a kind of canvas, created to be respected, certainly, but also to be transformed,” he told Helena Matheopoulos for her 1982 book “Maestro: Encounters With Conductors of Today.”

His emphasis on clarity, economy and objectivity was prized by the composers whose works he led. “What a refreshing change it is to hear a piece done so that you can grasp how it is written,” the British composer Peter Maxwell Davies told Matheopoulos.

Another French composer of an earlier and more conservative generation, Darius Milhaud, paid Mr. Boulez what must be the ultimate compliment. “He despises my music,” Milhaud once said, “but he conducts it better than anyone.”

Mr. Boulez was also renowned for his catalogue of compositions, some of which — the Sonatine for flute and piano (1946); the Piano Sonata No. 2 (1948); “Le Marteau sans Maitre” (1955) for alto and chamber ensemble, and “Pli Selon Pli” (1962) for soprano and large orchestra — counted among the most significant and influential works of their time.

The young Mr. Boulez referred to his music as “organized delirium,” and the phrase, rather than an oxymoron, was telling and apt. Mr. Boulez created a violent, glittering and emotional music that was, nevertheless, always carefully controlled, anchored by a highly complicated master plan.

In later years, Mr. Boulez was by all accounts a gracious, soft-spoken and self-effacing gentleman, much beloved by the musicians with whom he worked. In his composition and his conducting — which he managed with the brisk efficiency of a bank teller giving change — he was the antithesis of the romanticized stereotype of egoistic, heaven-storming musician.

“Perhaps I can explain it best by an old Chinese story,” he said to his biographer, the late Joan Peyser. “A painter drew a landscape so beautifully that he entered the picture and disappeared. For me, that is the definition of a great work — a landscape painted so well that the artist disappears in it.”

Pierre Boulez was born in Montbrison, France, on March 26, 1925. Although he sang in the choir of his Catholic school and was a skilled pianist, he originally planned to devote his career to engineering and mathematics.

Instead, he entered the Paris Conservatory in 1942 and began studies with Olivier Messiaen, who became his principal teacher. He graduated in 1945 at the top of his class, and by the time he was 25, he was already accepted as one of the brightest, fiercest — and most controversial — musical spirits in postwar Europe.

To many artists and intellectuals, World War II had seemed an absolute conclusion to “Old World” traditions, and the early Boulez was prone to sociocultural positions that were hardly less radical than his music.

For example, when the composer Arnold Schoenberg died in 1951, Mr. Boulez published a withering obituary dismissing most of the older man’s later work. It was titled “Schoenberg est mort!” (“Schoenberg is dead!”)

Mr. Boulez began his conducting career with a series of Parisian concerts he organized in the mid-1950s, titled “Domaine Musical” and devoted mainly to avant-garde music. Starting off conducting his own work, he moved gradually into the standard repertory and made his U.S. debut in 1964 with the Cleveland Orchestra; he would remain closely associated with the ensemble until his death.

By the time he agreed to take the New York Philharmonic position, he was recognized as one of the leading conductors of his time, a stature confirmed with clear, cool and revelatory recordings of works by Claude Debussy, Hector Berlioz, Igor Stravinsky and Bela Bartok.

“He built programs and seasons around themes that connected 18th and 19th century music with the 20th,” Los Angeles Times music critic Mark Swed wrote in 2010. “He expanded his audience by holding avant-garde ‘rug’ concerts in informal settings downtown to attract the young and unstuffy.”

Nevertheless, he came in for much criticism for what was perceived as his aloofness. The New York Times Magazine published a profile of Mr. Boulez in 1973; it was titled “The Iceberg Conducteth.”

His tastes were strict. “I hate Tchaikovsky, and I will not conduct him,” he said during his tenure at the Philharmonic. Still, he insisted that he was not a “fascist”: “If the audience wants him, it can have him.” He mellowed as he grew older, once saying that he now “enjoyed” listening to composers such as Tchaikovsky and Sibelius but still did not want to conduct them.

After he left New York in the late 1970s, the French government created the Paris-based Institut de Recherche et Coordination Acoustique/Musique (IRCAM), a sort of musical “think tank” and acoustical laboratory, to his specifications. He served as its director for the next two decades. In later years, he appeared mostly with the Chicago and Cleveland orchestras in the United States and the Vienna and Berlin philharmonics in Europe.

Mr. Boulez kept his private life resolutely private, with no acknowledged romantic relationship of any sort. Survivors could not be confirmed.

His recordings include Wagner’s 16-hour “Ring des Nibelungen” (which was also filmed for television); large works by Berlioz and Mahler, and a great deal of the music of Schoenberg, despite his early dismissal.

Some of Mr. Boulez’s less-predictable repertory choices included works by Anton Bruckner, Richard Strauss and some orchestral music by the rock musician and satirist Frank Zappa. Mr. Boulez made the first complete recording of Alban Berg’s opera “Lulu” and recorded the complete music of the Austrian composer Anton Webern twice over the course of three decades.

Mr. Boulez always considered himself primarily a composer, although his work tapered off sharply after he began to devote more time to conducting. His later pieces include “Eclat” (1965) for chamber orchestra; “Domaines” for clarinet solo or with 21 instruments (1968); “Repons” (1985); and a Piano Sonata No. 3, which was begun in the 1950s and has yet to be completed (portions of the score have been published and performed).

Perhaps his most telling title belongs to a work for chamber ensemble that was completed in 1973. It was titled “. . . Explosante/Fixe” — fixed explosion — an image that neatly summed up Mr. Boulez’s compositional ideal.

Page is a former classical music critic for The Washington Post.