The mega-cities of the developing world hold an irresistible attraction for migrants from the countryside. But as the cities swell, their need for food, water and energy multiplies. The BBC's Chris Morris asks whether urbanisation is sustainable.
Ujjwal is a 22-year-old labourer sitting outside a makeshift tent in a posh part of New Delhi. By day he works on repaving the road. By night he sleeps on the pavement.
Like millions of Indians, he has migrated to the big city to find work and earn money. It is the only way he can gain any benefit from the mainly-urban economic boom which has swept through this country.
"There are so many more opportunities available here than there are at home," he says.
"You have to work hard and the hours are long. But I don't want to go back to my village in Bengal. I want to stay here in Delhi."
Hundreds of thousands of migrant construction workers live in Delhi alone, many of them working on big marquee projects in advance of next year's Commonwealth Games.
Ujjwal earns about 5,000 rupees ($102) per month and manages to send at least 1,000 rupees home to his family.
But as more and more migrants arrive in Delhi, the pressure on land, on water supplies, and on urban infrastructure intensifies.
India's capital is creaking at the seams.
It's the same story across the developing world. Mega-cities have been growing at an incredible rate, and are struggling to cope with the demands of millions of new inhabitants.
Take the example of Mumbai. Its population has roughly doubled in the past 25 years, and millions of people live in the slums.
Ensuring access to clean water is a particular dilemma, and for many a daily grind.
Early in the morning, in north Mumbai, dozens of people are queuing in the rain with buckets and plastic jerry cans, hoping to buy water from a hose-pipe snaking out into the narrow street.
The municipal supply is only switched on for a couple of hours a day at best. So this is the one chance for people with no connection to the mains to get enough water to meet their basic needs.
"There's no planning whatsoever, and every day more and more people arrive in the city, and the problems keep getting worse," says Chandrashekhar Prabhu, a prominent architect.
But despite all the strains on the system there is an argument that migration from rural to urban areas in countries like India should increase still further.
A World Bank report earlier this year (The World Development Report 2009) says the process of migration should be welcomed and encouraged as a way of lifting people out of poverty.
"Instead of worrying about the size of metropolises, cities and towns, the government should worry about making sure that these places work well," said the director of the World Development Report, Indermit S Gill, on its release.
And that's a huge challenge.
Because if a decent percentage of this country's migrant workers succeed and settle down, they too will be seeking even more water, more food and more energy supplies.
India's cities are developing fast. But the demands of the people are developing faster.