Over the past decade, polycentric metropolitan areas have become an increasingly widespread but relatively new policy issue at the European, national and regional scale. In total 48,5% of the urban population in Europe lives in polycentric metropolitan areas. There are high expectations when it comes to the integrated development of polycentric metropolitan areas. So far, important knowledge questions as regards to the processes at play remain largely unanswered. EMI’s Knowledge and Research Agenda on ‘Polycentric Metropolitan Areas’ presents the main challenges for cities derived from the transition from single monocentric cities to polycentric metropolitan areas: entering in the upward spiral of metropolisation.