Over the past few decades, the population of Delhi has more than tripled, to 30 million—and the challenges of managing India’s largest metropolis have increased almost exponentially. In November, for instance, a spike in Delhi’s pollution maxed out air-quality monitors and caused flights to be diverted. Across the globe, rapidly growing megacities are grappling with similar problems, from overwhelmed sewers to crippling traffic to buckling infrastructure. According to the United Nations, the world’s urban population was 746 million in 1950, or about 30% of the total. Today some 4 billion people, more than 50% of the global population, live in cities. And by 2050, the UN projects, 68% of people will reside in urban areas. At right are the world’s 20 most populous “urban agglomerations,” as defined by the UN’s system of measuring extended city areas. Each is seeking the right blueprint for growth.

A version of this article appears in the March 2020 issue of Fortune with the headline “Plotting the Rise of the Megacity.”

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