A generation ago, New York and Tokyo were the world's only megacities. By 2025, the UN predicts there will be 37. All but a handful will be in the developing world. The fate of millions, then, rests on the question: what will life in these megacities look like? While density is an almost universally celebrated urban characteristic, rapid population growth can also result in poorly planned, congested, and unsafe settlements, leading many to ask: Are slums the inevitable urban form of the future? How might big data and progressive planning ensure that even the fastest growing cities are places of opportunity for the billions that will live in them? Luís Bettencourt, Shirish Sankhe, Kennedy Odede, Greg Lindsay