Johann Sebastian Bach (31 March [O.S. 21 March] 1685 – 28 July 1750) was a German composer and musician of the late Baroque period. He is known for his prolific output across a variety of instruments and forms, including the orchestral Brandenburg Concertos; solo instrumental works such as the cello suites and sonatas and partitas for solo violin; keyboard works such as the Goldberg Variations and The Well-Tempered Clavier; organ works such as the Schübler Chorales and the Toccata and Fugue in D minor; and choral works such as the St Matthew Passion and the Mass in B minor. Since the 19th-century Bach Revival, he has been widely regarded as one of the greatest composers in the history of Western music.

The Bach family had already produced several composers when Johann Sebastian was born as the last child of a city musician, Johann Ambrosius, in Eisenach. After being orphaned at age 10, he lived for five years with his eldest brother, Johann Christoph, then continued his musical education in Lüneburg. In 1703 he returned to Thuringia, working as a musician for Protestant churches in Arnstadt and Mühlhausen. Around that time he also visited for longer periods the courts in Weimar, where he expanded his organ repertory, and the reformed court at Köthen, where he was mostly engaged with chamber music. By 1723 he was hired as Thomaskantor (cantor with related duties at St Thomas School) in Leipzig. There he composed music for the principal Lutheran churches of the city and Leipzig University's student ensemble, Collegium Musicum. In 1726 he began publishing his organ and other keyboard music. In Leipzig, as had happened during some of his earlier positions, he had difficult relations with his employer. This situation was somewhat remedied when his sovereign, Augustus III of Poland, granted him the title of court composer of the Elector of Saxony in 1736. In the last decades of his life, Bach reworked and extended many of his earlier compositions. He died due to complications following eye surgery in 1750 at the age of 65. Four of his twenty children, Wilhelm Friedemann, Carl Philipp Emanuel, Johann Christoph Friedrich, and Johann Christian, became composers.

Bach enriched established German styles through his mastery of counterpoint, harmonic and motivic organisation, and his adaptation of rhythms, forms, and textures from abroad, particularly Italy and France. His compositions include hundreds of cantatas, both sacred and secular. He composed Latin church music, Passions, oratorios, and motets. He adopted Lutheran hymns, not only in his larger vocal works but also in such works as his four-part chorales and his sacred songs. Bach wrote extensively for organ and other keyboard instruments. He composed concertos, for instance for violin and for harpsichord, and suites, as chamber music as well as for orchestra. Many of his works use contrapuntal techniques like canon and fugue.

Several decades after the end of his life, in the 18th century, Bach was still primarily known as an organist. Several biographies of Bach were published in the 19th century, and by the end of that century all of his known music had been printed. Dissemination of Bach scholarship continued through periodicals (and later also websites) devoted to him, other publications such as the Bach-Werke-Verzeichnis (BWV, a numbered catalogue of his works), and new critical editions of his compositions. His music was further popularised by a multitude of arrangements, including the "Air on the G String" and "Jesu, Joy of Man's Desiring", and recordings, among them three different box sets of performances of his complete oeuvre marking the 250th anniversary of his death. By 2013, more than 150 recordings had been made of his Well-Tempered Clavier.