The United Nations estimates that at least one billion people—20% of the world's population—live in crowded, unsanitary slums of the central cities and in the vast shanty towns and squatter settlements that ring the outskirts of most Third World cities. Around 100 million people have no home at all. In Bombay, India, for example, it is thought that half a million people sleep on the streets, sidewalks, and traffic circles because they can find no other place to live. In São Paulo, Brazil, at least three million "street kids" who have run away from home or have been abandoned by their parents live however and wherever they can. This is surely a symptom of a tragic failure of social systems.

Slums are generally legal but otherwise inadequate multifamily tenements or rooming houses, either custom built for rent to poor people or converted from some other...