The slum question is not marginal to urban planning and social development – it is at its very heart. All the more so now that urban growth takes place primarily in developing countries in which populations move from rural to urban regions at a very fast pace. And 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. Sometimes this is due to a lack of political will, but more often it is the result of a lack of financial resources and insufficient or inappropriate individual and institutional capacities, with an impact in terms of inappropriate instruments and deficient support to decision-makers. 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. Through case studies in Latin America, Western Africa and South East Asia, we will demonstrate that the good practices in slum upgrading and urban development are generally based on similar criteria: a) an interdisciplinary approach of the issues linking environmental analysis, urbanism and socio-economic perspective; b) a multi-actors’ definition of strategies, policies and priorities in the implementation of concerted actions; c) a conception of urban planning, which includes sustainable development areas: sound territorial distribution of population and human activities; social equity; economic prosperity, and conservation of environmental resources; d) the translation of the expected balance into decision-making processes (institutional and political framework) and covering the economic, environmental and social costs to society (mainly the economic and financial dimensions).