From the sky, the collection of dwellings that make up a village, town or city is easily distinguished from more rural areas where people live a greater distance apart and where the pressure on local resources is less due to lower population densities. Forty-five percent of the world's total population lived in urban areas in 1995. Although this percentage was larger in developed countries than in developing ones, it is estimated that by 2025 more than two-thirds of the population of developing countries will live in cities. The environmental and social problems of large cities, and the largest of the world's cities known as megacities, are well-documented.
Sizable cities impose great environmental demands in terms of resource use and waste disposal.
Energy use, air and noise pollution, water supply, sanitation, and solid waste treatment are environmental problems which are often found in cities where the population numbers in the millions. Frequently, these conditions will be found in association with social problems such as poverty, unemployment and lack of access to basic services. It was in recognition of the growing development challenges in human settlements, especially as a consequence of rapid urbanization in the developing countries, that the United Nations convened the first global conference on human settlements in Vancouver in 1976. Throughout Agenda 21 linkages were identified between human settlements and a wide variety of other issues such as health, protection of the atmosphere, the marine environment, freshwater, solid wastes, children and youth, women, the scientific and technological community, education, public awareness and training, information for decision-making, business and industry, demographics, and poverty. Table 1 lists the chapters of Agenda 21 where objectives or activities relating to human settlements are found.