Arnold Schoenberg (13 September 1874 – 13 July 1951) was an Austrian and later American composer, associated with the expressionist movement in German poetry and art, and leader of the Second Viennese School. He used the spelling Schönberg until after his move to the United States in 1934, whereupon he altered it to Schoenberg "in deference to American practice", though one writer claims he made the change a year earlier.

Schoenberg's approach, both in terms of harmony and development, is among the major landmarks of 20th century musical thought; at least three generations of composers in the European and American traditions have consciously extended his thinking or, in some cases, passionately reacted against it. During the rise of the Nazi party in Austria, his music was labeled, alongside jazz, as degenerate art. Schoenberg was widely known early in his career for his success in simultaneously extending the traditionally opposed German Romantic styles of both Brahms and Wagner. Later, his name would come to personify pioneering innovations in atonality that would become the most polemical feature of 20th century art music. In the 1920s, Schoenberg developed the twelve-tone technique, a widely influential compositional method of manipulating an ordered series of all twelve notes in the chromatic scale. He also coined the term developing variation, and was the first modern composer to embrace ways of developing motifs without resorting to the dominance of a centralized melodic idea. Schoenberg was also a painter, an important music theorist, and an influential teacher of composition; his students included Alban Berg, Anton Webern, Hanns Eisler, and later John Cage, Lou Harrison, Earl Kim, and many other prominent musicians. Many of Schoenberg's practices, including the formalization of compositional method, and his habit of openly inviting audiences to think analytically, are echoed in avant-garde musical thought throughout the 20th century. His often polemical views of music history and aesthetics were crucial to many of the 20th century's significant musicologists and critics, including Theodor Adorno, Charles Rosen, and Carl Dahlhaus. Schoenberg's archival legacy is collected at the Arnold Schönberg Center in Vienna.

Arnold Schoenberg was born into a lower middle class Jewish family in the Leopoldstadt district (in earlier times a Jewish ghetto) of Vienna, at "Obere Donaustraße 5". Although his mother Pauline, a native of Prague, was a piano teacher (his father Samuel, a native of Bratislava, was a shopkeeper), Arnold was largely self-taught, taking only counterpoint lessons with the composer Alexander von Zemlinsky, who was to become his first brother-in-law. In his twenties, he lived by orchestrating operettas while composing works such as the string sextet Verklärte Nacht ("Transfigured Night") in 1899. He later made an orchestral version of this, which has come to be one of his most popular pieces. Both Richard Strauss and Gustav Mahler recognized Schoenberg's significance as a composer; Strauss when he encountered Schoenberg's Gurre-Lieder, and Mahler after hearing several of Schoenberg's early works. Strauss turned to a more conservative idiom in his own work after 1909 and at that point dismissed Schoenberg, but Mahler adopted Schoenberg as a protégé and continued to support him even after Schoenberg's style reached a point which Mahler could no longer understand, and Mahler worried about who would look after him after his death. Schoenberg, who had initially despised and mocked Mahler's music, was converted by the "thunderbolt" of Mahler's Third Symphony, which he considered a work of genius, and afterwards "even spoke of Mahler as a saint". In 1898 he converted to Lutheranism. He would remain Lutheran until 1933.

The summer of 1908, during which his wife Mathilde left him for several months for a young Austrian painter, Richard Gerstl (who committed suicide after her return to her husband and children), marked a distinct change in Schoenberg's work. It was during the absence of his wife that he composed "You lean against a silver-willow" (German: Du lehnest wider eine Silberweide), the thirteenth song in the cycle Das Buch der Hängenden Gärten, Op. 15, based on the collection of the same name by the German mystical poet Stefan George; this was the first composition without any reference at all to a key. Also in this year he completed one of his most revolutionary compositions, the String Quartet No. 2, whose first two movements, though chromatic in color, use traditional key signatures, yet whose final two movements, also settings of George, weaken the links with traditional tonality daringly (though both movements end on tonic chords, and the work is not yet fully non-tonal) and, breaking with previous string quartet practice, incorporate a soprano vocal line.

During the summer of 1910, Schoenberg wrote his Harmonielehre (Theory of Harmony, Schoenberg 1922), which to this day remains one of the most influential music theory books. From about 1911 Schönberg belonged to a circle of artists and intellectuals that included Lene Schneider-Kainer, Franz Werfel, Herwarth Walden and the latter's wife, Else Lasker-Schüler.

Another of his most important works from this atonal or pantonal period is the highly influential Pierrot Lunaire, Op. 21, of 1912, a novel cycle of expressionist songs set to a German translation of poems by the Belgian-French poet Albert Giraud. Utilizing the technique of Sprechstimme, or melodramatically spoken recitation, the work pairs a female vocalist with a small ensemble of 5 musicians. The ensemble, which is now commonly referred to as the Pierrot ensemble, consists of flute (doubling on piccolo), clarinet (doubling on bass clarinet), violin (doubling on viola), violoncello, speaker, and piano.

Wilhelm Bopp, director of the Vienna Conservatory from 1907, wanted a break from the stale environment personified for him by Robert Fuchs and Hermann Grädener. Having considered many candidates, he offered teaching positions to Schoenberg and Franz Schreker in 1912. At the time Schoenberg lived in Berlin. He was not completely cut off from the Vienna Conservatory, having taught a private theory course a year earlier. He seriously considered the offer, but he declined. Writing afterwards from Berlin to Alban Berg, he cited his "aversion to Vienna" as the main reason for his decision, while contemplating that it might have been the wrong one financially, but having made it he felt content. A couple of months later he even wrote to Schreker suggesting that it might have been a bad idea for him as well to accept the teaching position.

World War I brought a crisis in his development. Military service disrupted his life. He was never able to work uninterrupted or over a period of time, and as a result he left many unfinished works and undeveloped "beginnings". So, at the age of 42 he found himself in the army. On one occasion, a superior officer demanded to know if he was "this notorious Schoenberg, then"; Schoenberg replied: "Beg to report, sir, yes. Nobody wanted to be, someone had to be, so I let it be me" (Schoenberg 1975, 104) (according to Norman Lebrecht, this is a reference to Schoenberg's apparent "destiny" as the "Emancipator of Dissonance").

Later, Schoenberg was to develop the most influential version of the dodecaphonic (also known as twelve-tone) method of composition, which in French and English was given the alternative name serialism by René Leibowitz and Humphrey Searle in 1947. This technique was taken up by many of his students, who constituted the so-called Second Viennese School. They included Anton Webern, Alban Berg and Hanns Eisler, all of whom were profoundly influenced by Schoenberg. He published a number of books, ranging from his famous Harmonielehre (Theory of Harmony) to Fundamentals of Musical Composition (Schoenberg 1967), many of which are still in print and still used by musicians and developing composers.

Schoenberg viewed his development as a natural progression and he did not deprecate his earlier works when he ventured into serialism. In 1923 he wrote to the Swiss philanthropist Werner Reinhart: "For the present, it matters more to me if people understand my older works ... They are the natural forerunners of my later works, and only those who understand and comprehend these will be able to gain an understanding of the later works that goes beyond a fashionable bare minimum. I do not attach so much importance to being a musical bogey-man as to being a natural continuer of properly understood good old tradition!".

Following the 1924 death of composer Ferruccio Busoni, who had served as Director of a Master Class in Composition at the Prussian Academy of Arts in Berlin, Schoenberg was appointed to this post the next year, but because of health reasons was unable to take up his post until 1926. Among his notable students during this period were the composers Roberto Gerhard, Nikos Skalkottas, and Josef Rufer. Schoenberg continued in his post until the election of Adolf Hitler and the Nazis in 1933, when he was dismissed and forced into exile. He emigrated to Paris, where he is said to have acquired or formally reclaimed membership in the Jewish religion, and then to the United States. His first teaching position in the United States was at the Malkin Conservatory in Boston. He then moved to Los Angeles, where he taught at the University of Southern California and the University of California, Los Angeles, both of which later named a music building on their respective campuses Schoenberg Hall (UCLA Department of Music [2008]; University of Southern California Thornton School of Music [2008]). He settled in Brentwood Park, where he befriended fellow composer (and tennis partner) George Gershwin and began teaching at University of California, Los Angeles, where he resided for the rest of his life. Composers Leonard Rosenman and George Tremblay studied with Schoenberg at this time.

During this final period he composed several notable works, including the difficult Violin Concerto, Op. 36 (1934/36), the Kol Nidre, Op. 39, for chorus and orchestra (1938), the Ode to Napoleon Buonaparte, Op. 41 (1942), the haunting Piano Concerto, Op. 42 (1942), and his memorial to the victims of the Holocaust, A Survivor from Warsaw, Op. 46 (1947). He was unable to complete his opera Moses und Aron (1932/33), which was one of the first works of its genre to be written completely using dodecaphonic composition. In 1941, he became a naturalized citizen of the United States. During this period, his notable students included John Cage, Lou Harrison, and H. Owen Reed.

Schoenberg experienced triskaidekaphobia (the fear of the number 13), which possibly began in 1908 with the composition of the thirteenth song of the song cycle Das Buch der Hängenden Gärten Op. 15 (Stuckenschmidt 1977, 96). Moses und Aron was originally spelled Moses und Aaron, but when he realised this contained 13 letters, he changed it. His superstitious nature may have triggered his death. According to friend Katia Mann, he feared he would die during a year that was a multiple of 13. He so dreaded his sixty-fifth birthday in 1939 that a friend asked the composer and astrologer Dane Rudhyar to prepare Schoenberg's horoscope. Rudhyar did this and told Schoenberg that the year was dangerous, but not fatal. But in 1950, on his seventy-sixth birthday, an astrologer wrote Schoenberg a note warning him that the year was a critical one: 7 + 6 = 13. This stunned and depressed the composer, for up to that point he had only been wary of multiples of 13 and never considered adding the digits of his age. On Friday, 13 July 1951, Schoenberg stayed in bed — sick, anxious and depressed. In a letter to Schoenberg's sister Ottilie, dated 4 August 1951, his wife, Gertrud, reported "About a quarter to twelve I looked at the clock and said to myself: another quarter of an hour and then the worst is over. Then the doctor called me. Arnold's throat rattled twice, his heart gave a powerful beat and that was the end". Gertrud Schoenberg reported the next day in a telegram to her sister-in-law Ottilie that Arnold died at 11:45pm.