Metropolitan areas are continuously changing their spatial organisation, reflecting the evolution of economy and society. These changes affect the quality of life, the demand for transport infrastructure, and the global environmental footprint of urbanisation, among other factors. Regional, metropolitan and local governments’ decisions depend critically on the physical structure of the city. On average, 80% of the OECD urban population lives in the cores of metropolitan areas and only 20% in the hinterlands, but in a few European countries the share of population in urban cores is below 50% (). While most of the metropolitan areas have grown with contiguous urban cores, 30 metropolitan areas show a polycentric structure with more than one urban core.