The article reconstructs with the help of archival documents and contemporary maps the cartographic activity of the Belgian Institut Cartographique Militaire, predecessor of the current Institut Géographique National, during World War II. If the Institute left Belgium at the country’s invasion by the Germans in 1914, during World War II it remained in Belgium after a brief stay in France and resumed its cartographic activities. The analysis of the archival documents and maps shows how during the four years of occupation by the Germans, the German military administration of the country and the Institute, demilitarized and residing under the authority of the Commissariat Général à la Restauration du Pays, shared the Institute’s buildings, equipment and personnel in the framework of their respective mapping activities.