In this paper, we focus on the southern cross-border area between Spain and Portugal, which includes eighteen different municipalities on both sides of the border. We gathered information using a number of different techniques from a wide range of informants, including politicians, experts on cross-border cooperation, a broad range of civil society organizations and common citizens. Our objectives were to make a broad diagnosis of social reality in this area, to understand the dynamics of territorial development, including the possibilities and limitations of community development, and to obtain clear insight into the characteristics of the social networks that Portuguese and Spanish citizens are currently building. Our results contrast with existing assumptions on these issues, particularly regarding the relationships built by Spanish and Portuguese citizens, the social network arrangements they establish, the relative importance of the spaces of places and the spaces of flows and the sense of community on both sides of the border. We also question more general notions regarding the importance of cross-border cooperation, especially insofar as this concept usually emphasizes institutional relationships while overlooking non-formal and informal relationships between people. These latter relationships, we argue, may have greater potential to influence the sustainable development of the cross-border area between southern Portugal and Spain. We highlight the role of bottom-up activities that would promote closer relationships between Spanish and Portuguese citizens and communities and would change the typical social networks they establish.