The building process of cross-border territories in the EU is addressed in this paper with a geographical and political approach. The spatial evolution of institutionalized cross-border cooperation is investigated comparatively on two borders: the Cross-Channel Euroregion between France, Belgium and the United Kingdom, and the Upper Rhine region between France, Germany and Switzerland. Data collection on these case studies categorize the spatialization of cross-border cooperation. The links between the set of partners and the perimeter raise questions about the territorialisation process. The emergence, adjustments and even the disappearance of cross-border regions are examined, leading to a new quantitative and qualitative understanding of the spatial evolution of crossborder regions. Phenomena of juxtaposition and scaling fuel the multiplication of cross-border regions, while processes of shrinkage or disappearance are the exception. Adjustments of the cross-border perimeter are primarily due to enlargement, expansion and fusion. A few cross-border cooperation initiatives are developing a new relationship to their cross-border region, distinguishing the institutional perimeter of the partnership from the functional perimeter of projects.In an uncertain European context, cross-border regions tend to multiply and extend. Showing similarities with 'soft spaces', they prove themselves to be very adaptable in their spatial (re-)definition.