The speed with which many of the world´s cities are growing affects their social and physical structure dramatically. The five cities featured in this section - Cairo, Istanbul, London, Los Angeles and Shanghai - are experiencing growth at a different pace, and absorbing it in different manners: some are growing upwards, others are growing outwards.

The speed of urban change is fuelled by natural increase as well as rural-to-urban migration, with people flocking to cities in search of jobs and social opportunities. Some cities, like London, are managing this change with planned strategies for growth. Others are overtaken by informal development processes that completely transform their scale, texture and character.

In many cases, the social and urban landscapes of these cities are being rapidly transformed by the speculative commercial development of business parks, commercial districts, shopping malls and housing enclaves. These changes directly affect the lives of existing communities and new city residents; the way in which they are managed is central to the creation of more balanced and inclusive cities in the twenty-first century.
http://www.tate.org.uk/modern/exhibitions/globalcities/speed.shtm