Aside from the growth of world population itself, urbanization is the dominant demographic trend of the late twentieth century. The number of people living in cities increased from six hundred million in 1950 to over two billion in 1986. If this growth continues unabated, more than half of humanity will reside in urban areas shortly after the turn of the century. Resultant signs of urban stress now apparent around the world call into question the continuing expansion of cities. Land and water scarcity, inefficient energy use and waste disposal, and the resultant problems of pollution all contribute to the escalating ecological and economic costs of supporting modern cities. Accelerated urbanization in the Third World has spurred the concentration of political power within cities, leading to policies that favor urban over rural areas. Now, mounting external debts are forcing Third World governments to scale back urban subsidies just as the demand for services multiplies. Today's urban areas, larger and more numerous than ever, have outgrown the capacity of natural and social systems to support them. One way of reaching a nation's optimum rural-urban balance would be for a market economy to play a more prominent role in economic development. Chapters include: "The Growth and Role of Cities"; "Urban Energy Needs"; "Feeding Cities"; "Nutrient Recycling"; "Ecology and Economics of City Size"; and "Seeking a Rural-Urban Balance." (BZ)