Beyond Contractualism tackles the problem of how to sustain an international order when power is redistributing itself and when a normative consensus among states can no longer simply be assumed. Recourse to violence cannot solve the problem of order durably and neither can recourse to reasoned persuasion or contractual pacts. Between violence and reason lies the concept of authority and deference to authority has indeed often been presented as the solution to the problem of order, including of international order. But if authority is the solution, it nonetheless begs the question: how does international authority become established and how is it maintained. This project addresses these questions through a comparison of how the Achaemenid Empire, Medieval Christendom, and today's Global International Society have each grappled with their 'problem of international authority.'