The developing world already packs 2.6 billion people into its relatively dense cities. In 100 years, it could have three times as many urban residents. As their per capita income grows, they will they will also demand more land, perhaps twice as much per person as they do today. Governments can accommodate this increased demand either with a sixfold increase in the average built area of existing cities or with a combined strategy of expanding existing cities and developing entirely new cities. The Commissioners’ Plan of 1811, which guided a sevenfold increase in the built area of New York City, shows that a government can manage successful urban expansion on the required scale if it implements a plan that is narrow but strong. China’s development of Shenzhen shows that a government can use a new city to unleash systemic reform. The next few decades offer a unique opportunity to speed up progress by following these examples.