Reich and Glass became notorious in their early days for writing minimal, repetitive music, but music that was strangely hypnotic for all that. This is still an inherent feature of much that they do and it has had an influence in both pop music and contemporary music. The use of a steady pulse, something that had become anathema to modernist composers after WWII, made their music highly listenable for all audiences. But both Reich and Glass have moved far beyond the earlier, simpler music to highly complex structures, but ones still anchored in a rhythmic base.
One of the characteristics that makes the music of earlier composers like J. S. Bach and Beethoven so compelling is the sense of inevitability: their music has a direction, a journey, to it. This sense of inevitability was something that seemed to get sacrificed in the avant-garde music of the 20th century. Whatever we were told or whatever we knew about the structure of the music, it just sounded arbitrary and fleeting. A lot of this was because of the non-repetitive, jagged rhythms.
So what Steve Reich did, inspired by non-Western music such as the drumming of Ghana, was to rediscover rhythm in its most pure form. Here is the first part of his long piece Drumming performed by the Portland Percussion Group:
Though it is incomplete (the next section is on marimbas), it is an excellent video as it shows something of how the piece is put together. Essentially, he takes a simple pattern in 6/4 and reveals it, element by element. Then he shifts the patterns against one another. He plays with our perceptions. Inevitably, confronted with one beat, we hear it as the downbeat, so when the real downbeat arrives, it is quite a surprise.
Steve Reich has built considerably on these early ideas. Drumming dates from 1971 while the next piece, Sextet, dating from 1984, adds a lot of harmonic interest. This is the Yale Percussion Group in a complete performance:
Percussionists love his music because, for the first time in music history, they are the ones leading the way!