There has been a recent trend in the literature on committees in Westminster parliaments to evaluate their performance through their impact on different groups such as the government, media and the parliament. This paper uses this literature to develop a formal framework for evaluating committees. It argues that, due to their political role, the performance of committees should be evaluated in terms of political impact or influence, rather than the audit or programme-based concept of effectiveness. A committee can demonstrate that one of its activities or reports has performed positively if at least one of its relevant groups demonstrates approval of it. The relevant groups for parliamentary committees are the government, the bureaucracy, the legislature, external stakeholders, the public, and the judiciary.