Based on a theoretical corpus derived from sociology and urban geography, this article proposes to define the concept of commons. In order to formulate this conceptual proposal, the article is first based on a critical analysis of the literature on cross-border integration, in particular, the field of border studies. As a counterpoint to functionalist approaches to integration, the article proposes to use the concept of commons to develop a new approach in analysing cross-border relations; it, thus, puts into perspective the importance of attachments, social ties, and identities in the modalities of living together in cross-border regions. The conceptual proposal developed in this article is supported by three case studies that present specific and contrasting cross-border relations. These are Greater Geneva, the Lille-Kortrijk-Tournai Eurometropolis, and the Basque Eurocity.