What makes a great city? It is a pressing question because by 2030, 5 billion people—60 percent of the world’s population—will live in cities, compared with 3.6 billion today, turbocharging the world’s economic growth. Leaders in developing nations must cope with urbanization on an unprecedented scale, while those in developed ones wrestle with aging infrastructures and stretched budgets. All are fighting to secure or maintain the competitiveness of their cities and the livelihoods of the people who live in them. And all are aware of the environmental legacy they will leave if they fail to find more sustainable, resource-efficient ways of managing these cities.