Current population: 16.7 million
In some ways, the growth of Shanghai tells the story of contemporary China. In 1980, it had a population of less than 6 million and was not among the top 15 global cities. In 30 years, it has nearly tripled in size to 16.7 million and leaped to the ninth spot on the list. In 2010, the city became the busiest container port the world, surpassing Singapore. It is also a burgeoning financial hub, drawing migrants from across the region. To cope with rapid growth, the government has invested in massive infrastructure projects, including the subway system, which is now the largest in the world.