By 2050, 70 percent of the world's population will live in cities (UN-HABITAT 2008). These cities will have to provide services and resources to their inhabitants while reducing pollution. Sustainability policies designed by planners and public actors are supposed to meet this challenge, but what will be the interaction between the implementation of these policies and social justice? This chapter focuses on the sharp processes of spatial differentiation and the many-fold conflicts and tradeoffs between sustainabilityas related in the dominant discourse only to efficiency and resource conservationand social justice. It considers the Paris metropolitan region (known as Ile-de-France), a major urban area in Europe. The spatial divide in the Ile-de-France has increased significantly during the last twenty years between wealthy and poor people as well as between ethnic groups, though the existence of these divides is not officially recognized in France (Burgel 2009). This spatial separation coincided temporally with the rising of sustainability policies in the Ile-de-France after 2001. As we will later see, differential exposure to environmental nuisances is a key factor in this divide. In the Paris metropolitan region, as well as in the rest of French cities and urban areas, there was no real environmental concernwhether in the population or the local authoritiesbefore the end of the 1970s (Larrère and Larrère 1997). In Europe, authoritarian planning and massive construction of high-density housing developments marked the aftermath of the Second World War. Besides, in 1965, Ile-de-France's master plan organized the extension of the city of Paris from an Combining Sustainability and Social Justice