Now that real-time computer music systems are relatively portable and affordable, it has become common to use them in live performance. Unfortunately, most computer music systems offer little more than a tape recorder in terms of their abilities to interact with live musicians. Computer accompaniment was designed to go beyond this “tape recorder” model of human-computer interaction. Computer accompaniment is a process in which a computer “listens” to a live musician, follows along in a score, and synchronizes an “accompaniment” score with the live player. A system has been implemented that can reliably accompany a live musician in spite of tempo changes and wrong notes. Input is acoustic for instruments that produce a single tone (monophonic) and via mechanical sensors for keyboard input. The talk will focus on the pattern matching used to follow the score and on tempo adjustment techniques used to produce a musical accompaniment.