Professor Guy Sinclair explains how the UN has, over time, diverged from its original Charter, adapting in response to global shifts—sometimes enhancing its relevance, but at other times straining its legitimacy. Sinclair outlines four key factors driving institutional change: external shocks, internal dynamics, legal mechanisms, and shared narratives. As new technologies like AI and autonomous weapons reshape global threats, he anticipates further evolution in how international organizations operate. We also discuss the strain on the UN’s development agenda—especially the Sustainable Development Goals—as geopolitical shocks and donor fatigue undercut progress. Sinclair addresses concerns about the UN’s diminished authority in peace and security, citing the growing irrelevance of Security Council authorization. He warns that the increasing role of private and state donors may skew UN priorities, challenging its legitimacy. Finally, we explore whether collective security remains a viable goal and whether reform should be gradual or transformative. Through it all, Sinclair urges a critical but hopeful approach, recognizing the UN’s enduring potential amid deep systemic challenges.