The phenomenon of urban brownfields is significant throughout European metropolitan areas. In this chapter, we assess, both in qualitative and quantitative terms, the inherent potential of urban brownfields to provide a relevant and substantial densification strategy for metropolitan areas. First, we explore the various opportunities for improvement of the built environment offered by urban brownfields in terms of environment, society, and economics, which are the core principles of sustainable development. This analysis is, inter alia, a juxtaposition between urban brownfields and the compact and polycentric city model, adapted to the metropolitan area. While brownfield regeneration appears to be a relevant densification strategy, it nevertheless implies that a sufficient reserve of land is available to engage policymakers. Thus, we attempt to estimate the urban brownfield stock in three countries: the United Kingdom, Switzerland, and France. The data subsequently serves as a basis for our calculation of the theoretical construction potential of brownfield sites.