Book section: The United Nations Human Settlements Programme (UN-Habitat, 2003) estimated that in 2001, 924 million people, about 32% of the world's urban population, lived in slums. This figure could reach 1.7 billion by 2020–and 2.8 billion by 2030 (Lopez Moreno, 2003). Case studies in developing countries usually estimate that between 20 and 90% of a city's population live in informal or illegal settlements. These figures are debatable, given the criteria for defining slums (UN-Habitat, 2003). Still, they suggest the magnitude of the problem and its alarming dynamics.