Human settlements have been both the home and the hallmark of civilization since the first human beings congregated. This history of each part of the Earth is told through the cultural, economic, and social settings of cities, towns, and villages. City states continue to this day, and often the commercial and political life of large cities defines the policies of states. The world's cultural heritage is bound up in its human settlements. So, too, is the world's future.

In the latter years of the 20th century, the emergence of megacities signaled a reshaping of all aspects of both national life and international relations. There are models of planned urban growth, such as the brilliant development of the Pudong New Area in Shanghai, China, or the transformation of Singapore after the Second World War into a clean and green city, with extraordinary provisions guaranteeing the well-being of its citizens. These examples demonstrate that the environmental and social and economic pillars of sustainable development can be coordinated and advanced in tandem. Unfortunately, these examples are the exceptions. Many of the megaconurbations of millions of city inhabitants lack clean water, sewage systems, decent housing, educational opportunities, jobs, and parks and recreation. Despite major social and urban planning programs in cities across Brazil, favelas persist and grow in many states. Slums and shantytowns are a defining feature of major cities in many African, Asian, and South American nations.