Augmenting performances of live popular music with computer systems poses many new challenges. Here, "popular music" is taken to mean music with a mostly steady tempo, some improvisational elements, and largely predetermined melodies, harmonies, and other parts. The overall problem is studied by developing a framework consisting of constraints and subproblems that any solution should address. These problems include beat acquisition, beat phase, score location, sound synthesis, data preparation, and adaptation. A prototype system is described that offers a set of solutions to the problems posed by the framework, and future work is suggested.