In Britain we are often told that people are leaving the big cities to live in the countryside but is this the case worldwide? Not at all. If you look at the biggest cities in 1950, seven out of the top ten were in the developed countries but by the year 2000, the developing countries will have eight out of the top ten. New York, which in 1950 was number one with a population of around 12 million, will only be the sixth largest city in the world but with an extra 2 million. And London? London, which was number two, won't even be in the top ten. Its population in 1950, by the way, was about 10 million. And why is this happening? Why are people moving to the big cities from the country in developing countries? The reasons are complex but many are moving to look for work. And the problems this creates are enormous. It's estimated that 26 million people will be living in Mexico City by the year 2000, with Sao Paulo in Brazil not far behind.