Bela Bartok was a Hungarian composer, pianist and collector of Eastern European and Middle Eastern folk music. He is considered one of the greatest composers of the 20th century and was also one of the founders of the field of ethnomusicology, the study and ethnography of folk music.

Bartok was born in the small Transylvanian town of Nagyszentmiklos in Austria-Hungary (now Sannicolau Mare, Romania). He displayed notable musical talent very early in life: according to his mother, he could distinguish between different dance rhythms that she played on the piano even before he learned to speak in complete sentences. By the age of four, he was able to play 40 songs on the piano, and his mother began formally teaching him the next year.

Bartok was a small and sickly child, and suffered from a painful chronic rash until the age of five. In 1888, when he was seven, his father (the director of an agricultural school) died suddenly, and Bartok's mother then took him and his sister Erzsebet to live in Nagyszolos (today Vinogradiv, Ukraine), and then to Pozsony (today Bratislava, Slovakia). In Pressburg bartok gave his first public recital at age eleven to a warm critical reception. Among the pieces he played was his own first composition, written two years previously: a short piece called "The Course of the Danube." Shortly thereafter he was accepted as a student of Laszlo Erkel.

Bartok studied piano under Istvan Thoman (a former student of Franz Liszt) and composition under Janos Koessler at the Royal Academy of Music in Budapest from 1899 to 1903. There he met Zoltan Kodaly, who would influence him greatly and who become his lifelong friend and colleague. In 1903, Bartok wrote his first major orchestral work, Kossuth, a symphonic poem which honored Lajos Kossuth, hero of the Hungarian revolution of 1848.

It was the music of Richard Strauss, whom he met at the Budapest premiere of Also sprach Zarathustra in 1902, that had the most influence on his early work. From 1907 his music also began to be influenced by the music of Claude Debussy that Kodaly had brought back from Paris. His large scale orchestral works were still in the manner of Johannes Brahms or Richard Strauss, but also around this time he wrote a number of small piano pieces which show his growing interest in folk music. Probably the first piece to show clear signs of this new interest is the String Quartet No. 1 in A minor (1908), which has a few folk-like elements in it.

In 1907, Bartok began teaching as a piano professor at the Royal Academy. This position freed him from touring Europe as a pianist and enabled him to stay in Hungary. Among his notable students were Fritz Reiner, Sir Georg Solti, Gyorgy Sandor, Erno Balogh, Lili Kraus, and, after Bartok moved to the United States, Jack Beeson.

Bartok also made a lasting contribution to the literature for younger students: for his son Péter's music lessons, he composed Mikrokosmos, a six-volume collection of graded piano pieces. It remains popular with piano teachers today.

In 1908, inspired by both their own interest in folk music and by the contemporary resurgence of interest in traditional national culture, he and Kodaly undertook an expedition into the countryside to collect and research old Magyar folk melodies.

In 1909, Bartok married Marta Ziegler. Their son, Bela Jr., was born in 1910.

In 1911, Bartok wrote what was to be his only opera, Bluebeard's Castle, dedicated to his wife, Marta. He entered it for a prize awarded by the Hungarian Fine Arts Commission, which rejected it out of hand as unplayable. In 1917-1918, Bartok completely reworked the score of the opera, and then he was pressured by the government to remove the name of the librettist, Bela Balazs, from the program on account of his political views. Bartok refused, and eventually withdrew the work. For the remainder of his life, although he was passionately devoted to Hungary, its people and culture, he never felt much loyalty to its government or official establishments.

After his disappointment over the Fine Arts Commission prize, Bartok wrote very little for two or three years, preferring to concentrate on collecting and arranging folk music (in Central Europe, the Balkans, Algeria, and Turkey). However, the outbreak of World War I forced him to stop these expeditions, and he returned to composing, writing the ballet The Wooden Prince in 1914–16 and the String Quartet No. 2 in 1915–17, both influenced by Debussy. It was The Wooden Prince which gave him some degree of international fame.

Raised as a Roman Catholic, Bartok had by his early adulthood become an atheist, decrying the existence of God as unknowable and unnecessary. He would later become attracted to Unitarianism, and publicly converted to the Unitarian faith in 1916. His son would later become president of the Hungarian Unitarian Church.

He subsequently worked on another ballet, The Miraculous Mandarin influenced by Igor Stravinsky, Arnold Schoenberg, as well as Richard Strauss, following this up with his two violin sonatas (written in 1921 and 1922 respectively) which are harmonically and structurally some of the most complex pieces he wrote. The Miraculous Mandarin was started in 1918, but not performed until 1926 because of its sexual content, a sordid modern story of prostitution, robbery, and murder. He wrote his third and fourth string quartets in 1927–28, after which he finally found his true voice, starting broadly incorporating folk music in his compositions. Notable examples of this period are Divertimento for strings (1939) and Music for Strings, Percussion and Celesta (1936). The String Quartet No. 5 (1934) is written in somewhat more traditional style. Bartok wrote his sixth and last string quartet in 1939.

Bartok divorced Marta in 1923, and married a piano student, Ditta Pasztory. His second son, Peter, was born in 1924.

In 1940, as the European political situation worsened after the outbreak of World War II, Bartok was increasingly tempted to flee Hungary.

Bartok was strongly opposed to the Nazis. After they came into power in Germany, he refused to give concerts there and switched away from his German publisher. His liberal views caused him a great deal of trouble from conservatives in Hungary.

Having first sent his manuscripts out of the country, Bartok reluctantly emigrated to the USA with Ditta Pasztory, settling in New York City. Peter Bartok joined them in 1942 and later enlisted in the United States Navy. Béla Bartok, Jr. remained in Hungary.

Bartok did not feel comfortable in the USA, and found it very difficult to write. He was also not very well known in America, and there was little interest in his music. He and his wife Ditta would give concerts, and for a while, they had a research grant to work on a collection of Yugoslav folk songs. While their finances were always precarious, it is a myth that he lived and died in poverty. There were enough supporters to see to it that there was enough money and work available for him to live on. Bartok's health began to deteriorate in 1944, making composing difficult for him. His last work might well have been the String Quartet No. 6, were it not for Serge Koussevitsky commissioning him to write the Concerto for Orchestra, at the behest of the violinist Joseph Szigeti and the conductor Fritz Reiner (who had been Bartok's friend and champion since his days as Bartok's student at the Royal Academy). This quickly became Bartok's most popular work, and one which would ease his financial burdens. He was also commissioned by Yehudi Menuhin to write a Sonata for Solo Violin. This seemed to reawaken his interest in composing, and he went on to write his Piano Concerto No. 3, an airy and almost neo-classical work, and begin work on his Viola Concerto.

Bartok died in New York from leukemia (as a result of secondary polycythemia) on September 26, 1945 at age 64. He left the viola concerto unfinished at his death; it was later completed by his pupil, Tibor Serly.

He was interred in the Ferncliff Cemetery in Hartsdale, New York, but during the gradual fall of Hungarian communism in the late 1980s, his remains were transferred to Budapest, Hungary for a state funeral on July 7, 1988 with interment in Budapest's Farkasreti Cemetery.