Given today’s unprecedented level of global interconnectedness, multilateral institutions have become increasingly important, despite detractors’ legitimate concerns over their effectiveness. But corruption, lack of legitimacy, poor management, and ill-defined missions in many current multilateral institutions diminish their ability to facilitate cooperation and communication among countries interested in the pursuit of shared objectives. This paper examines the Group of 20 (G20), calling attention to its strengths as a relatively new, unburdened, and nimble organization but also noting widespread concerns about the legitimacy of its membership. To bolster the group’s legitimacy, the paper outlines a set of mission-based, quantitative criteria on which G20 membership could be based. After assessing the impact on the G20 of applying these criteria, the paper concludes by recommending purpose-driven criteria as a framework for new multilateral organizations, to both promote effectiveness and bolster confidence in their legitimacy.