Cities are increasingly at the centre of global flows of people, capital, culture and information. Over the last 30 years their role as financial command centres has expanded, creating a new type of sprawling, often multi-centred, urban agglomeration.

There are now over 20 mega-city regions with more than ten million people. There are also nearly 450 city regions with over one million residents. Together they house more than one billion people in a relatively small surface of the earth. As they expand even further, into urbanised regions of over 50 million inhabitants, their footprint will have a direct impact on climate change and the ecological balance of the planet, as well as on the lives of existing and new city dwellers. This section explores some of the most populous city regions of the world – the greater Tokyo area (the largest urban region in the world today), and the expanding metropolitan zones of Mexico City and São Paulo. Each city displays different spatial characteristics and varying levels of success in managing urban change through governance and policies to contain sprawl. Some of these policies, such as London’s Green Belt, established by Patrick Abercrombie in 1943, can have a lasting impact on the city’s ecology and liveability.