Cities, not nation-states, are the dominant unit of human organization in the twenty-first century. Humanity has shifted from a predominantly rural to urban species in a startlingly short period of time. The world today is stitched together by thousands of small, medium, and large cities—including 31 mega-cities, depending on how you define them—that are dramatically transforming our political, social, and economic relations. Yet, despite the centrality of cities in modern life and to resolving critical global challenges, our international affairs are still dominated by nation-states. This status quo is no longer acceptable.