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Megacities have particular significance in this world-wide process of urbanisation. New scales have evolved ("mass matters"), including new dimensions of large high-density concentrations of population with immense sprawl and a serious increase in infrastructural, socio-economic and ecological overload. Furthermore, these may develop extreme dynamism in demographic, economic, social and political processes. Both phenomena - the new scale and dynamism - make megacities vulnerable, especially where administrative direction is absent or weak.
According to different definitions, megacities are on a purely quantitative level those metropolises with a population of over 5 M, more than 8 M or more than 10 M inhabitants. Some authors also set a minimum level for population density (at least 2,000 persons/km2) and only include cities with a single dominant centre, whereby polycentric agglomerations such as the Rhein-Ruhr area in Germany, for example, with 12.8 M inhabitants, are excluded. Others include this polycentric mega-urban region (UN 2002, pp. 116-118, see central map of documentation on this website). Ultimately it is futile to fight over a fixed definition of megacities, as any setting of minimum/maximum values is subjective and thus open to debate. Furthermore there are the problems of inconsistent spatial boundaries for administrative districts, as well as the reliability of up-to-date population figures given inconsistent censuses, projections and estimations. International statistics are not based on similar areas of reference, so that the figures given for the size of cities and megacities are generally not comparable.
While in the 1950s there were only four cities with a population greater than 5 M, by 1985 there were already 28 and in 2000 39. Depending on the threshold accepted as a lowest population value for a megacity, there are currently worldwide 16, 24 or 39 megacities; in the year 2015 there will probably be almost 60. Before World War II megacities were a phenomenon of industrialised countries; today by far the greater number are concentrated in developing countries and Newly Industrialising Countries (NICs). Two thirds of the megacities are now in developing countries, most of them in East and South Asia. At the moment just under 394.2 M people live in megacities, 246.4 M of them in developing countries, more than 214.5 M in Asia. In 2015 there will be about 604.4 M people living in megacities. In some of these - Mexico City, São Paulo, Seoul, Mumbai, Jakarta and Teheran - the population figures have almost trebled between 1970 and 2000.
Megacities have a large number of specific problems with occasionally striking structural similarities. Among the most important common characteristics are high population concentration and density, with extreme levels in some cases, largely uncontrolled spatial expansion, high traffic levels, in some cases severe infrastructural deficits, high concentrations of industrial production, signs of ecological strain and overload, unregulated and disparate land and property markets and insufficient housing provision, in some cases extreme socio-economic disparities - as well as a high level of dynamism in all demographic, social, political, economic and ecological processes. One must nevertheless be wary of generalizing statements, for among megacities there are clear differences in infrastructural quality, the level of economic development (e.g. transformation processes), social polarisation or political leadership and governability, differences which should not be ignored.