The aim of this research project is to build knowledge about the critical mass for the Metropolitan Core area. In planning policy was also highlighted in the past on the spatial coherence in some of the RSV: called "urban network at international level, the Flemish Diamond". However, the element of critical mass has so far not explicitly recognized in spatial policy. The delineation of example both the Flemish Strategic Area around Brussels and the Metropolitan Area Antwerp was found insufficient space to achieve the targets. Then were to achieve these remaining tasks. Then the remaining tasks were transferred to the provinces of Flemish Brabant, respectively. Antwerp, who sought space for this in their small urban areas, regardless of their location in or outside the Flemish Diamond. This took "mass" on the Flemish Diamond. The project Metropolitan Core area we would just reinforce the critical mass. An important aspect is to find sufficient densification opportunities in urban regions that make up the Metropolitan Core area. This requires a mechanism between the various metropolitan regions in order to ensure that critical mass. This can for instance be envisaged transfer mechanisms, the "trade" package housing between the urban regions in the core, etc.