As global governance institutions appear increasingly contested by state and non-state actors alike, understanding their origin, operation, and impact is becoming ever more urgent. This symposium uses Michael Zürn's A Theory of Global Governance: Authority, Legitimacy, and Contestation (OUP, 2018) as a springboard to explore the state of global governance theory. A Theory opens new terrain and advances bold and original arguments, including the contention that global governance is itself best understood as a political system. It analyzes a cycle from rising authority beyond the state through the 20th century, to ensuing legitimation problems toward the century's end, to the politicization and contestation triggered by such problems. A book of such ambition inevitably elicits queries within diverse international relations research communities. This symposium features seven articles from diverse traditions in engagement with A Theory's understanding of global contestation, authority, and legitimacy. These are followed by a response from Zürn. An introduction situates A Theory within extant research on global governance, highlights its endogenous theory of global politics, and identifies the stakes of deepening research on the sources of global authority, contestation, and political legitimation.