The prosperity and environmental sustainability of cities are inextricably linked. Urban areas consume huge amounts of environmental goods and services like food, water, energy, forestry, building materials, and ‘green’ or open spaces often beyond their boundaries. This undermines the assimilative capacity of the natural environment around urban areas. 1 For example, the cities of the world generate over 720 billion tons of wastes every year, but in developing regions, even in large, presumably more affluent cities, only 25 to 55 per cent of wastes are collected. 2 Demographic and spatial expansion can be so rapid as to outstrip the capacity of cities to provide basic amenities − housing, water and sanitation, etc. − resulting in poor urban conditions. 3