The project The cross-border metropolis hypothesis: Back to the source is a visiting research proposal supporting the secondment of one of LISER’s researchers, Dr. Christophe Sohn, at the following institutions:•Center for U.S.-Mexican Studies based at the University of California San Diego School of Global Policy and Strategy, the most prominent policy research institute on the U.S.-Mexico border in North America (0.8 full-time equivalent, FTE)•Department of Geography, University of California Los Angeles, one of the highest ranked departments of geography in the world (0.2 FTE)The objectives of this visiting research initiative are threefold. They comprise (i) research activities, (ii) capacity building and (iii) networking.Research activities are the primary focus of the intended visit. Two projects will be pursued during the 12 months foreseen for the stay. They build upon and develop Christophe Sohn’s research agenda which focuses on cross-border metropolises, taking advantage of the expertise of and collaboration with resident UCSD/UCLA researchers as well as the relevance of the location of San Diego for building up a grounded research. The first project takes stock of the visiting researcher’s work on the role of borders as a resource and seeks to explore the virtues of a Europe-North America dialogue that aims at context-sensitive comparative analysis. The second project intends to assess the process of the social production of the cross-border metropolitan space through a triadic approach sensitive to the multiplicity of bordering dynamics and inspired by the work of Lefebvre.A second purpose of this visit is to further enhance the research potential and outreach of the visiting fellow (and by extension of his research group in Luxembourg) through (i) integration within the thriving academic community in San Diego and Los Angeles, and (ii) the building of a grounded expertise on cross-border urban and regional planning and economic development in a comparative perspective. The relevance of these research areas for the department of Urban Development and Mobility at LISER, which focuses most of its research on the processes of cross-border metropolitan integration underscores the collective added value of this visit in terms of research expertise and international reputation. Finally, and in line with the foregoing, the secondment will be an opportunity to consider future joint projects and establish long-term partnerships between LISER and key US academic partners in the field of border studies framed in relation to a comparative dialogue.