Most of these megacities are in developing countries that are struggling to cope with both the speed and the scale of human migration. Estimates of the future spread of urbanization are based on the observation that in Europe, and in North and South America, the urban share of the total population has stabilized at 75%--85%. If the rest of the world follows this path it is expected that in the next decade an extra 100 million people will join the cities of Africa, and 340 million the cities of Asia: the equivalent of a new Bangkok every two months. By 2030 nearly two-thirds of the world's population will be urban.
In the long run, that is good news. If countries now industrialising follow the pattern of those that have already done so, their city-dwellers will be both more prosperous and healthier. Man is gregarious species, and the Words "urbane" and "civilised" both derive from the advantages of living in large settlements.
History also shows, though, that the transition can be uncomfortable. The slums of Manchester were, in their time, just as awful as those of Nairobi today. But people moved there for exactly the same reason: however nasty conditions seemed, the opportunities of urban life outstripped those of the countryside. The question is how best to handle the change.
If there is one thing that everybody agrees on, it is that urbanization is unstoppable. Migrants attempting to escape poverty, and refugees escaping conflict, are piling into cities in what the executive director of UN-HABITAT, Anna, Tibailjuka, describes as "premature urbanization."
Dr Tibaijuka believes it might be possible to slow the pace of migration from the countryside with policies that enhance security and rural livelihoods. There is room for debate, though, over whether better rural development in any form. can seriously slow the pace of urbanisation -- or even whether such a slowdown would be a good thing.
Michael Mutter, an urban planning adviser at the British government's Department for International Development (DFID), says that the relevant indicators suggest that in many countries the effective "carrying capacity" of rural areas has been reached. As happened in Europe in the 18th century, population growth and technological improvements to agriculture are creating a surplus population. That surplus has to go somewhere to earn its living.
Indeed, some people go so far so to argue that governments, international donors and aid agencies spend too much on rural development and neglect the cities. Most countries have a rural development policy, but only a few have urban ones. DFID, for example, spends only 5% of its budget directly on urban development. Moreover, these critics point out that, although rural areas often have worse sanitation, illiteracy and homelessness than cities, such figures are deceptive. Being illiterate, homeless or without access to a flush toilet are far more serious problems in a crowded city than in the countryside.
Of the many lessons being learnt from past urban-development failures, one of the most important is that improvements must involve local people in a meaningful way. Even when it comes to the poorest slumdwellers, some governments and city authorities are realizing that people are their own greatest assets. Slumdwellers International is a collection of "grassroots" federations of people living in slums. Its idea is simple. Slum-dwellers in a particular place get together and form. a federation to strengthen local savings and credit schemes, and to lobby for greater co- operation with the authorities. Such federations are having a big impact

A. the side effects of urbanization
B. megacities in developing countries
C. the causes behind immigration to cities
D. ways to slow down the pace of immigration