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The number of urban dwellers is expected to hit 5 billion globally by 2025--double the number of 1990, according to studies by the World Resource Institute. As it turns out, many people just like city life, and telecommuting opportunities are limited: Doing business still requires face time. Here's a look at the likely candidates for 2025's 10 biggest.

Tokyo, Japan

Current Population: 35.2 million

Projected 2025 Population: 36.4 million

Annual Growth Rate 2007 to 2025: 0.11%

The world's largest city now and (probably) in the future, Tokyo is a major global financial center. Its rail system, the largest in the world, is clean and efficient. Tokyo's water and electricity capacity is in fine shape for a growing population, experts say, but industrial and household waste removal is a growing concern. Per capita tonnage has doubled in the past 20 years, and finding urban space to build more processing plants has been a challenge.