(CNN) -- Just over 9 percent of the world's urban population lives in a megacity today.
By 2025, the number of megacities with a population greater than 10 million will grow from 21 to 29 and account for 10.3 percent of the world's urban population, according to the United Nations.
Here we highlight some of the world's megacities, such as the most populous: Tokyo in Japan, which, with 35.2 million inhabitants, has a larger population than Algeria, Canada or Uganda; and the rapidly expanding Shanghai, China's economic powerhouse, which is the world's biggest metropolitan area, covering 2,914 square kilometers.
Megacities can bring not just wealth and employment but also cultural benefits, says the United Nations.
And some megacities are more efficient than their less populous counterparts. While New York houses 2.7 percent of the U.S. population, New Yorkers only account for one percent of the country's CO2 emissions.
However, the high density of people can generate problems, including health issues such as the rapid transmission of 'flu viruses and psychological problems caused by poverty and stress.
But whatever we think of them, megacities are not only here to stay: They're growing. New Delhi, for example, is projected to have 29.49 million inhabitants by 2025, according to Demographia -- up from 20.99 million in 2010.