The UN estimates that half of the world's population, 3.2 billion, now live in cities, compared with 13% (0.2 billion) in 1900, 29% (0.7 billion) in 1950, and 37% (1.5 billion) in 1975.

The world is undergoing a period of unprecedented change, characterised by a rising population, rapid economic growth, the spread of globalisation, pressure on the environment and continuing social, political and cultural divisions.