They've driven culture forward, facilitated global commerce, and charged ahead of their nation-bound peers.

Indeed, cities?—?and the metropolitan regions that orbit around them?—?make sense as a political and economic unit. The key services we depend on government to do, from building infrastructure to ensuring public safety, are mostly handled by cities. And contrary to earlier predictions, the forces of globalization and the rise of the information economy have only made cities more important as economic engines and innovation hubs. It's no surprise, then, that cities?—?and their mayors?—?are increasingly finding their voices in a world previously dominated by nations and international entities.