The pipe organ at Notre-Dame de Montréal Basilica, Montreal. Most of the pipes are behind shutters hidden by the visible pipes. These shutters regulate the volume. Many such large installations will have a secondary set of pipes in the choir loft, at the opposite end of the interior space.

A pipe organ is a keyboard instrument that makes sound by forcing air through large wood or metal pipes. Pipe organs are commonly found in churches and there is a large repertoire of religious music for the pipe organ. Many composers, famously including the Baroque composer Johann Sebastian Bach, have written extensively for the pipe organ. Music written specifically for the pipe organ is common from the Renaissance to the present day; north Germany is particularly notable for having produced many composers for the instrument.

Pipe organs are complex musical instruments, with hundreds or thousands of pipes of varying size and pitch. Controls on the console of the organ called stops select which pipes are used; different combinations of stops can change the timbre of the instrument considerably. Many pipe organs are mammoth instruments that are part of the church building itself.

Pipes may be classified in a number of different ways: by the material they are made of (wood or metal), by the mechanism of sound production (flue pipes vs. reed pipes, also called labial and lingual), by the shape of the pipe, and by the construction of the ends (open or closed). Each variation results in a different timbre.

Because a pipe produces only one pitch at a time, ideally there is at least one pipe for each controlling key or pedal. (Occasionally some pipes, especially in the bass, to save space or material, are rigged to provide multiple pitches like big recorders: this method was employed especially by a few builders in the early 20th century.) Thus, a keyboard with 61 notes should have 61 pipes. A complete set of pipes producing different pitches of one timbre is called a rank.

The pitch produced is a function of the length of the pipe, and many timbres are associated with ranks pitched some multiple of octaves apart: thus an organ stop may have similar names with the addition of a length in feet indicating the pitch: a 16' stop produces pitches an octave below that of an 8' stop, an 8' stop produces pitches an octave below that of an 4' stop, and a 4' stop produces pitches an octave below that of an 2' stop. Non-integral lengths (e.g. 2 2/3') are also quite widespread; these mutations produce sounds at pitch intervals other than octaves, and are generally used to provide colourful effects. This works by reinforcing certain partials of the overtone series of a fundamental; normally these mutation stops would not be played by themselves.

Some timbres require more than one pipe per key. This is often reflected in the name given to the stop as a Roman numeral: thus a stop called "Cornet V" on a 61 note manual (this is the usual number on U.S. organs) would have 5 × 61 = 305 pipes.

In some organs the extreme bass stops in the pedal department, usually represented by 32' or 64', may not contain "genuine" sounding pipes. This is usually for practical reasons such as cost or space, which may prohibit the provision of very large pipes. In such cases the sound is approximated by using harmonics. For example, a note on a 32' stop can be approximated by combining the equivalent 16' note with the note a tenth above it (known as a "quint"). The resultant beat frequency gives a passable impression of a 32' note. The effect is less good (and less necessary) for higher pitches where the ear's frequency response is better.

The pipe organ has at least one keyboard, with 2-5 keyboards being the most common configuration. Each keyboard is called a "manual", so that an organ with four keyboards is said to have four manuals. Most pipe organs also have a set of keys played by the feet called "pedals". The manuals, pedals and stop controls are gathered together in a functional piece of furniture called a "console".

From the time of the organ's invention by the ancient Greeks until the 19th century, pipes were originally located within a cabinet or "case", with the console and related mechanism built in. The valves under the pipes, called pallets, were connected by mechanical linkages to the keys, so that the organist's fingers actually provided the energy to open the pallets. This system is known as "mechanical (or "tracker") key action".

With the invention of electrical and pneumatic control systems in the late 19th century, organ pipes were often located remotely from the console in special rooms called chambers. In the 1920s and '30s, there was a revival of interest in historic organs, and organ builders once again began building organs with mechanical action. Today, both electric action and mechanical action pipe organs are built.

The largest pipe organ ever built, containing more than 32,000 pipes, is the Main Auditorium Organ in Atlantic City Convention Hall , built by the Midmer-Losh Organ Company between 1929 and 1932. The second largest, with over 28,000 pipes, is the Grand Court Organ at Wanamaker's department store (now Lord and Taylor) in Philadelphia.

Electronic organs such as the Hammond Organ were originally developed as imitations of pipe organs.

Notable organ builders:

• Aristide Cavaillé-Coll (France)
• François-Henri Clicquot (France)
• Arp Schnitger (Germany)
• Gottfried Silbermann (Germany)
• Rudolf von Beckerath (Germany)
• Henry "Father" Willis (England)
• Holtkamp Organ Company , Cleveland, Ohio
• Wicks Organ Company, Highland, Illinois
• Ernest M. Skinner (USA)
• M.P. Moller (USA)

Pipe Organs are related with the Gothic culture of the middle ages, and especially with Gothic cathedrals. In modern music, pipe organ sound (most often produced by electronic organ) are sometimes used by goth metal bands.