The Helikopter-Streichquartett (English: Helicopter String Quartet) is one of Karlheinz Stockhausen's best-known pieces, and one of the most complex to perform. It involves a string quartet, four helicopters with pilots, as well as audio and video equipment and technicians. It was first performed and recorded in 1995. Although performable as a self-sufficient piece, it also forms the third scene of the opera Mittwoch aus Licht ("Wednesday from Light"), from the opera cycle Licht.

The Helicopter Quartet was originally commissioned by Hans Landesmann of the Salzburger Festspiele in early 1991. Stockhausen's initial reaction was that he was not interested in writing a string quartet, but then one night he dreamed he was flying above four helicopters, each carrying a member of a string quartet; he could see into and through the transparent helicopters. He subsequently made some sketches and plans, but it was not until 1992–93 that he found the time to compose the quartet. By this time, he had had several more dreams concerning the piece, including one involving a swarm of bees and a violinist, about which Stockhausen said, "The buzzing made by lots of bees is a magic sound to me". The Arditti Quartet was to play the première. After Stockhausen finished his score, it was sent back to Landesmann for criticism. His reaction was positive, as was that of the Director of the Festspiele, Gerard Mortier. A long series of negotiations started with the Festspiele and the Austrian army, who were to lend the helicopters, as well as various TV channels who were airing the piece. In part because of protests by the Austrian Green Party, that it would be "absolutely impossible for Austrian air to be polluted by performing this Stockhausen", in the end the planned 1994 première had to be cancelled.

The first performances of the piece took place in Amsterdam on 26 June 1995, as part of the Holland Festival, with Alouette helicopters from the Royal Dutch Air Force display team, the Grasshoppers [nl]. The performers were: first violinist Irvine Arditti and pilot Marco Oliver; second violinist Graeme Jennings and pilot Lieutenant Denis Jans; violist Garth Knox and pilot Lieutenant Robert de Lange; cellist Rohan de Saram and pilot Captain Erik Boekelman. There were three performances given, at the Westergasfabriek, after two test flights at the same location the day before, and several earlier at an airfield in Deelen for the purpose of trying out the microphones. Following these performances, Stockhausen revised the score, adding about three minutes of material near the end, just before the descent, increasing the overall duration from about 29 minutes to 32 minutes. Since its premiere, the Helicopter Quartet has been performed "fairly regularly" and has become "the most iconic piece of classical music from the 1990s", though it was not presented in its full context, as the third scene of Mittwoch aus Licht, until the opera's staged premiere in August 2012.

In 2001 Angelin Preljocaj choreographed this music for a modern dance work titled Helikopter.

The Licht cycle—as a whole and in all of its parts—is serially composed, being based upon a three-layered "superformula". This design principle is an extension of the series-based techniques Stockhausen developed at the beginning of the 1950s—a connection to which Stockhausen himself has repeatedly called attention in his theoretical explanations. The Quartet is based on a short portion of the Eve layer from the Wednesday segment of the superformula: D–F–G♭. After the initial ascent, there are three iterations of the three-voiced superformula, each time distributed among the four instruments, and each time with the Michael, Eve, and Lucifer lines in a different vertical arrangement. The superformula as a whole is successively transposed so that the Eve layer begins each time with the corresponding pitch from the "helicopter" segment: D–F–G♭. The durations of the three statements of the triple formula are also determined by the rhythms of the small "helicopter" segment.

More general traits found in the work are also characteristic of the serial fabric of Stockhausen's music. For example, the interpenetration of macro and micro dimensions, found in earlier compositions such as Kontakte, where Stockhausen compresses rhythm into pitch, or in Hymnen, where he slows down the sound of geese until they are revealed as the shouts of a football crowd. The violin tremolos here and also in Mittwoch's preceding scene, Orchester-Finalisten, invoke the sound of a buzzing mosquito, so "what the composer is also saying is that the mosquito is also a tiny helicopter", and the connection between the two is being made by the violin. Another is the way in which the scenic character of the Helicopter Quartet forms one of four "serial variants": The first and fourth scenes of the opera represent the idea of communication and cooperation, first when World Parliamentarians meet to debate the topic of love, and then when interplanetary delegates consider cosmic problems, while the second scene and this one revolve around the idea of community music making.

A third serial principle is the integration of distinct elements into a whole. This was expressed by Stockhausen in a text written in 1953:

Evidently, self-contemplation and the awareness of a universal, planned order exist today "more than ever". With this comes the desire to give the individual tone a very specific sense that transcends momentary saturation and the merely impulsive play of organization and combination; a sense, that is, of music as a representation of that comprehensive "global" structure in which everything is integrated.