The nightmare of the not-too-distant future could be Cairo or Jakarta or any of a dozen other urban monsters that loom just over the demographic horizon. Already Mexico City, Sao Paulo, and Shanghai are among the largest, most congested cities on earth. Over the next two decades, they – and many others – are expected almost to double in size, generating economic and social problems that will far outstrip all previous experience.Just 30 years ago some 700 million people lived in cities. Today the number stands 1’900 million, and by the end of the century it will top 3’000 million – more than half the world’s estimated population.The flood of “urbanities” is engulfing not the richest, but the poorest. By the year 2100 an estimated 750 million people will crowd into 60 cities of five million or more – three-quarters of them in the developing world. Only a single First World city – metropolitan Tokyo, which will have 24 million people – is expected to be among the global top five; London, ranked second in 1950 with then ten million inhabitants, will not even make 2000’s top 25.What confronts and confounds urban planners is the enormity of these trends. There have never been cities over 30 million people, let alone ones dependant on roads, sewer and water supplies barely adequate for urban areas a tenth that size.The great urban industrial booms of Europe and America in the nineteenth and twentieth centuries sustained the cities that they helped to spawn. But in today’s swelling Third World cities, the flood of new arrivals far outstrips the supple of job – particularly sea modern industries put premium on technology rather than o n manpower. So it will be virtually impossible to find permanent employment for 30 to 40 percent of the 1’000 new city dweller expected by the year 2001.Optimists maintain that making rural or small-town life more attractive can stem urban growth. Some say that the trends is self correcting, since conditions will eventually get bad enough to convince people that the city life is no improvement at all. But pessimists see a gloomier correction: epidemics, starvation and even revolution. In the end, both sides agree that the world’s biggest cities are mushrooming into unknown.