The path-breaking achievements of the past century offer grounds for cautious hope. Over the coming decades, economic and technological interdependence will continue to intensify, binding the interests of the world’s peoples even more tightly together than today. As nations and regions find themselves increasingly “in the same boat,” win-win solutions among them are likely to become more self-evidently attractive. To be sure, the possibility of failure is ever-present and very real. Building more effective instruments for planet-level governance will prove exceedingly hard, and we can expect plenty of disheartening setbacks along the way. But the evidence from the past, and the plausible trajectory of challenges and incentives that await us in the coming century, suggest that it can be done.