International audienceThis debate specifically focuses on densification as a particular dimension of (post-) suburbanization. In the introduction, we discuss densification, along with ‘compactness’ and ‘intensification’, conceptual terms that have become buzzwords within urban planning. Objectives associated with these tend to be presented in the literature within a normative framework, structured by a critique of the negative effects attributed to sprawl. The perspective here is different. It is not normative but critical, and articulated around the analysis of political and social issues, related to the transformation of wider metropolitan space. Three main themes are developed: (1) the politics of densification (the environmental arguments favouring densification are highly plastic, and are thus often used to defend projects or initiatives which are actually determined by other agendas); (2) why morphology matters (a similar number of houses or square metres can be established in many different ways, and those different ways have political and social meaning); (3) the diversity of suburban densification regimes (it is not only the landscapes of the suburbs that are diverse, but also the local bodies governing them—between the small residential municipalities of the Paris periurbs and the large inner suburbs of Toronto lies a broad spectrum)

How do households belonging to the middle classes decide to come live in a 'poor'city in the Parisian suburbs? What makes them stay? What are the judgements and strategies that have been brought to bear both individually in their daily lives and as a social group in terms of their collective involvement and their relationships with other social groups? What does this kind of 'social mix' imply in terms of social practices, local and social belonging? This article shows that attitudes towards social mixing have to be considered in terms of their sociological dimensions. This analysis thus takes into account middleclass diversity in socio-residential terms, in terms of trajectories, values and attachmentto a particular area. The article also emphasizes the importance of distinguishing between different dimensions of attitudes towards social mixing: depending on the issues at stake, people can accept and sometimes promote a form of cohabitation, or, quite to the contrary, reject it. Finally, the article emphasizes the importance of local contexts and their temporality: for someone belonging to the middle classes, living in a poor city may also mean holding a relatively higher position than elsewhere.