Four dimensions of the concept of polycentricity (morphological, functional, relational and political polycentricity) are discussed in this paper based on the study of the Paris city-region / Bassin parisien system. The concentration of 'abstract production' workers in the Paris agglomeration and the fairly concentrated geography of firms' information flows (as measured by their phone calls) prevent the formation of an enlarged polycentric Mega-City-Region. Three explanations are proposed based on the strategies of real estate developers, information-intensive firms and policy-makers. In conclusion, I try to evaluate in the Paris city-region and French context whether a limited polycentricity at Bassin parisien level is prejudicial at different policy relevant scales.