Cities are the driving force of social and economic development. They harbour tremendous energies and have the potential to generate enormous creativity and significant economic betterment. They provide shelter, jobs and services and are the centres of productivity. For this reason, they attract more and more people and at present absorb two thirds of all population growth while generating over half of the Gross National Product (GNP), even in countries where the majority of the population is engaged in agriculture. People who live in cities expect clean water and adequate sanitation; they count on city authorities to ensure the efficient collection and disposal of domestic and industrial refuse; and they require good roads and a satisfactory transport system. But with the start of the new millennium, what characterises many of the world cities is uncollected garbage and filth, poor drainage and impassable roads, unsafe water supplies, inadequate infrastructure, polluted rivers and fouled air. These problems lead to, and are exacerbated by, reduced living standards and increased costs, growing disease and loss of productivity. The unprecedented pace at which many cities are growing, particularly in developing countries, outstrips their existing capacity for effective urban planning and management. The resultant degradation of the urban environment hinders the full realisation of the contribution cities can make to development, by threatening economic efficiency, social equity and the sustainability of hard-won development achievements.