Current population: 20.6 million
Home to 20.6 million people, the Seoul metropolitan area owes its gargantuan size to its inclusion of surrounding areas like the Incheon metropolis and the Gyeonggi province. The South Korean capital is highly developed and plans to offer free wi-fi in outdoor spaces around the city by 2015. But its advanced infrastructure belies the typical urban problems that are magnified because of its size. Its high population density makes for severe air pollution and smog, overtaxed sewers, crowded public transportation and an affordable-housing shortage.