This paper describes a computer accompaniment system capable of providing musical accompaniment for an ensemble of performers. The system tracks the performance of each musician in the ensemble to determine current score location and tempo of the ensemble. "Missing parts" in the composition (i.e., the accompaniment) are synthesized and synchronized to the ensemble. The paper presents an overview of the component problems of automated musical accompaniment and discusses solutions and their implementation. The system has been tested with solo performers as well as ensembles having as many as three performers.