The slum question is at the very heart to urban planning and social development. The urban growth takes place primarily in developing countries. Around one billion of people (with a perspective of 2 billion in 2030), the third of the world’s total urban population, live in such conditions of precariousness. Confronted with this demographic and spatial revolution, urban decision-makers are often helpless. The aim of this communication is to: 1) actualize the analysis of the phenomenon; 2) outline through a number of urban case studies in several emerging and developing countries, some relevant conditions that should be gathered to improve the human and built situation with a view to urban sustainable development.