Hypnotic music marked by very few contrasts and pioneered by La Monte Young
in the 1950s. Young's music is characterized by long tones (e.g. Trio for Strings) to the point of drone
s (e.g. his work with the Theatre of Eternal Music
) and just intonation
(most notably in The Well-Tuned Piano). Terry Riley
worked with Young and adopted a style similar to his in the early 60s, but soon after began working in tape music (pioneering process music) and highly modular music (e.g. his hyper-famous In C
Steve Reich soon adopted/adapted Riley's tape-music techniques (see It's Gonna Rain and Different Trains) and began working in modules, as did his peer Philip Glass, who defined most people's concept of minimal music in the 1970's---much to the chagrin of composers less willing to go so heavily mainstream. Reich was also popular in the 1970s, and John Adams in the 1980s. Glass's best-known work is Einstein on the Beach, Adams's probably Short Ride in a Fast Machine. Meanwhile, back in the late 1960s to early 1970s (and, of course, later), lesser-known (but hardly less brilliant) composers working in the minimalist style produced such haunting work as Strumming Music and Schlingen-Blängen (Charlemagne Palestine), The Four-Note Opera (Tom Johnson), I am sitting in a room (Alvin Lucier), among, of course, many others (including the work of Tony Conrad and John Cale of The Velvet Underground, both one-time members of the Theatre of Eternal Music, as was Angus MacLise, the Velvet Maureen Tucker replaced).
Some of Morton Feldman's work can also be considered minimalist, and the same holds for Pauline Oliveros. Pretty much all were influenced in some way by the work of John Cage, though aleatory processes don't lend themselves particularly well to music like that of Young (and Conrad, and Palestine, and probably so on)--while other work (e.g. some Riley's and Glass's modular music, Reich's Pendulum Music) is somewhat random by nature.