Ktesibios of Alexandria, in the 3d cent. B.C., invented the hydraulis, in which water pressure was used to stabilize the wind supply. The pipes were arranged in rows upon the wind chest and the air was permitted to enter any pipe at will by means of wooden sliders. The hydraulis was the prevailing organ for several centuries and reappeared at intervals throughout the Middle Ages.

Evidence of the first purely pneumatic organ is found on an obelisk erected at Byzantium before A.D. 393. Byzantium became the center of organ building in the Middle Ages, and in 757 Constantine V presented a Byzantine organ to Pepin the Short. This is the earliest positive evidence of the appearance of the organ in Western Europe. By the 10th cent., however, organ building had made considerable progress in Germany and England. The organ built c.950 in Winchester Cathedral is said to have had 400 pipes and 26 bellows and required two players and 70 men to operate the bellows.

The keyboard, or manual, was a creation of the 13th cent., making possible the performance of more complex music. The earliest extant music written specifically for organ, dating from the early 14th cent., gives evidence that by then the manuals of the organ had full chromatic scales, at least in the middle registers. Organs in the Middle Ages already had several ranks of pipes, each key causing a number of pipes to sound simultaneously. All were diapasons, or principals, the pipes of timbre characteristic only of the organ, and the various pipes controlled by one key were tuned to the fundamental and several harmonics of a given tone.

The 15th cent. saw considerable development of the organ, particularly in Germany and Flanders. It became possible to sound single pipes from a rank through the use of stops. Mutation and mixture stops that produce several harmonics of the unison pitch came to be used in combination with the unison to vary tone color. Solo stops imitative of other instruments, mainly flute and reed pipes, were added, and the pedal became standard. Until the 19th cent., Italy and England preferred an organ with no pedals.

It was the Flemish and German builders who developed the organ of distinctive and contrasting timbres, and the peak in organ building was reached in the German organ of the baroque, as described by Michael Praetorius in his Syntagma musicum (1618). The greatest organ builder, perhaps of all time, was Gottfried Silbermann (1683–1753) of Dresden. His organs produced a light, transparent tone, ideal for the performance of the great baroque polyphonic music. After this period the art of organ building degenerated, and the organ lost its place in the center of musical life.

The 19th-century desire for a highly expressive organ led to the obscuring of diapason tone by the large number of stops imitative of orchestral tone and to the common employment of the swell and the crescendo pedal. The swell involves enclosing one or more divisions of the organ in a wooden box on one side of which are shutters opened or closed by means of a swell pedal; the crescendo pedal, when gradually opened or closed, adds or takes off stops one by one.

The early 20th cent. saw the electrification of the mechanical parts of the organ, fulfilling the trend toward monstrous size and overwhelming power. In America, this large king of instruments became a feature of municipal auditoriums, movie palaces, churches, department stores, schools, and many other institutions. The master architect of these colossal orchestral organs was Ernest M. Skinner. In the early 20th cent., however, Albert Schweitzer was active in the preservation and restoration of many fine old organs, and there was a movement back to the ideals of Silbermann. In the United States, Walter Holtkamp, beginning in 1932, and G. Donald Harrison, in 1935, became the leading figures in this movement. Harrison designed many organs suitable for the performance of music of all periods. In the United States much of the repertoire was performed by the two leading organists of the era, E. Power Biggs and Virgil Fox. By the beginning of the 21st cent., European and American organ builders continued to concentrate on early principles for the construction of their instruments.

The organ repertory is vast and varied. The great organ masterpieces of the 17th and 18th cent. include works by John Bull, Handel, Jan Sweelinck, Girolamo Frescobaldi, and Dietrich Buxtehude. In the compositions of J. S. Bach the capabilities of the organ found their most magnificent expression.