The American composer Steve Reich turns 70 on 23 October and celebrations to mark the occasion are taking place worldwide, including a festival at the Barbican Centre in London and the release of a CD box set, Phases. Reich's standing is such that he has been dubbed America's - even the world's - greatest living composer. Lavish claims have been made about him having altered musical history. His signature style, which is essentially repetitive yet constantly morphing, has indeed been enormously influential. It has touched the modern classical composers Louis Andriessen and Michael Nyman; jazz and rock groups such as the Chicago Underground Duo and Tortoise; and electronic music from Nineties techno to experimental artists such as Alec Empire and Brian Eno.
Reich's finest compositions are visionary: Drumming (1971) used acoustic sound sources - drums, tuned percussion, piccolo and voice - but it was an uncanny prediction of the structure and rhythmic relentlessness of techno forms. Other works still slip seamlessly into DJ sets and have been sampled by all and sundry, from The Orb to the Japanese ambient artist Susumu Yokota. Others have enthusiastically reworked Reich compositions - 1999's Reich: Remixed has been followed by Reich: Remixed 2006 (released 4 September), which the composer helped in compiling. Remixing Reich, the 7 October show at the Barbican, will be an evening of live remixes by DJ Spooky and Coldcut.
Whether Reich continues to exert the kind of musical influence he did early on in his career, however, is another matter. Although it seems somewhat churlish to criticise a composer whose work is based on repetition for having a tendency to repeat himself, he has been so widely imitated that some of his latest compositions seem somewhat less than revolutionary.
His greatest musical achievement is his 1976 work Music for 18 Musicians. This hour-long piece, ostensibly based on a single rhythmic pulse, develops as a series of cells within which the musicians play gradually augmented figures. Not an enticing proposition on paper, perhaps, but the combination of multiple pianos dancing around each other, strings, fluttering voices and shuddering bass clarinet is ineffably beautiful. This composition was a defining moment in Reich's career - and casts a long shadow over his subsequent work.
In contrast, The Cave (1993), an ambitious multimedia collaboration with his wife, Beryl Korot, is one of his few genuinely tedious musical statements. The Phases box set demonstrates how much variety there has been in his work over the past four decades: from Different Trains, a powerful synthesis of speech samples and string quartet, to Proverb (1995), a gorgeous small-scale piece for organ, vibraphone and voice whose only lyrics are a quotation from Wittgenstein ("How small a thought it takes to fill a whole life"). But although he is still busily composing, one can't imagine him making any grand valedictory statements like the Ninth Symphonies of Beethoven, Bruckner or Mahler.
Perhaps this is in part because his music has been appropriated so extensively by younger artists. When I first interviewed Reich a decade ago, we talked about The Orb sampling his multiple-guitar piece Electric Counterpoint for their single "Little Fluffy Clouds". He joked that he'd told his lawyers not to sue them because they wouldn't have enough money. At the time, I wondered if this response was somewhat forced. But when I spoke to him again last year, he reiterated that he was genuinely flattered by the attention. He refused to differentiate between people actually sampling his recordings and the more general quoting of musical material that has gone on for centuries.
Reich's spirit of generosity will be an enduring legacy. His project as a young composer was to unite "the serious music and the trash out there in the street", as he describes it. As a student in New York in the very early Sixties, he rebelled against academic orthodoxy, which had erected a wall between the two. Influenced by the arrival of Arnold Schoenberg and the serialist Viennese School of the early 20th century, this cerebral approach deliberately removed composition from the popular forces that had fuelled it for centuries. It was, in Reich's opinion, "totally artificial and out of kilter with the history of western music". Part of his musical project has been to restore some "state of normalcy".
In his openness to diverse and sometimes conflicting influences, he prefigured many of the DJs and samplers of latter years. Although Reich was obliged to write serialist music while a student at the Juilliard, he was also playing gigs as a jazz drummer and was fired up by Stravinsky, as well as the flowing lines and melodic canons of J S Bach. "You can play Bach with a drummer, but you ain't gonna play no Mahler with a drummer," he has since joked. He was also attracted to the unchanging bass riff in Junior Walker's "Shotgun", the virtually single-chord blues of Bob Dylan's "Maggie's Farm" and John Col trane's more modal explorations.
Add to that an interest in the clear, long-held vocal lines of early music, African drumming - which Reich studied at the University of Ghana in Legon, Accra, in 1970 - and the dazzling tuned-percussion ensembles of Balinese gamelan, and you find an impressive array of elements informing his approach.
Reich's earliest compositions are rigorous process and conceptual pieces. In "Pendulum Music", from 1968, the sound source is feedback caused by three or more microphones being swung across tape recorders. (Seemingly of its time, it was recorded again by Sonic Youth in 1999.) Come Out, from 1966, is another process piece, in which two tape recorders playing the same loop of speech gradually run out of phase with each other, to mind-boggling effect.
A comparison with other, similar minimalist composers shows that Reich has undoubtedly stood the test of time better than most. In common with many other musicians who have been so labelled, Reich dislikes the term "minimalist". But he does concede that he shared a reductive approach to music with composers such as Philip Glass and Terry Riley back in the Sixties. This was synchronous, at least, with the activities of New York-based minimalist visual artists such as Sol LeWitt and Dan Flavin.
The perennial depiction of Reich and Glass as minimalism's Big Two caused strains in their friendship, which led on to rivalry. Whereas Reich's compositions since the end of the Eighties still sound relatively lithe and limber, Glass's orchestral and choral pieces have traded his dazzling rhythmic fluidity of yore for a sort of lumpen grandiosity. Reich came similarly unstuck with one of his few orchestral works, The Four Sections (1987) - a sort of "Young Minimalist's Guide to the Orchestra", and one of his less inspired efforts.
In recent years, Reich has become absorbed in pursuing what he describes as his "assignment". Since his marriage to Korot in 1976, he has become increasingly immersed in Judaism and some of his compositions have taken explicitly political themes. His latest work, Daniel Variations, to be premièred at the Barbican, celebrates the life of Daniel Pearl, the American journalist murdered in Pakistan in 2002. Reich has scored text from the Book of Daniel and the journalist's own words: "My name is Daniel Pearl. I am a Jewish American." He may no longer be a musical revolutionary, but Steve Reich is still pursuing a unique vision.
The season Phases: the music of Steve Reich is at the Barbican Centre, London EC2, until 8 October. For tickets and more details visit: www.barbican.org.uk
Reich: a brief guide
It's Gonna Rain (1965) was Reich's first major work. He took the recording of a ranting Pentecostal preacher, duplicated it and then played it "out of phase" with itself. He compares the technique to that of Bach's fugues. It could also be described as proto-hip-hop, its repeated "scratch" sound prefiguring New York DJs and their backing tracks for rap artists.
Music for 18 Musicians (1976) features only 11 chords, but it took Reich two years to write the piece, and it is a milestone for him and contemporary music. It was this piece that the agents provocateurs/ remix masters Coldcut chose to fiddle with in homage to the composer. Their version is available on Reich: Remixed.
Desert Music (1984) was inspired by Stravinsky's Rite of Spring, a work that changed Reich's attitude to music for ever. In recognition of the influence of the Russian composer's work on him, the London Symphony Orchestra will perform both Desert Music and The Rite of Spring at the Barbican on 1 October.
Electric Counterpoint (1987) is a soundscape of 11 guitars layered on top of each other. The Orb stole shamelessly from it for a sample for their 1991 ambient hit "Little Fluffy Clouds".
Different Trains (1988) took Reich in a new direction. A fiercely emotional work of startling originality, it focuses on his personal experience of travelling by train between the east and west coasts of America as a child in the Forties, shuttling between his separated parents. Using sampled sounds and voices, he compares his experience to the transportation of Jews by rail to the death camps during the Second World War.
The Cave (1993) is an epic, multimedia opera, created in collaboration with his wife, the video artist Beryl Korot. It explores the figures of Abraham in the Old Testament and the Torah and Ibrahim in the Koran, quoting Palestinians, Israelis and Americans in its libretto.