Minimalism was the last great revolution to happen in the world of art music. Young American composers began experimenting with using limited materials and processes in the 1960s, and the result was music that relied on repetition and very slow change over time.
Music for 18 Musicians from 1976 by Steve Reich:
The extremely repetitious patterns and simplicity of harmony and melody are a big departure from the kind of European classical music taught at conservatories; the listener is given the opportunity to take their time, to wander, to become entranced,
Critics have called minimalist music primitive, commercial and emotionless; I think the real reason for the reaction is that these composers dared to use sounds and processes from popular music, raising the question of what would happen to our musical institutions if the boundary between popular and art music were to get uncomfortably fuzzy?
Music: Music For 18 Musicians by Steve Reich