The same characteristics that make cities socially and economically progressive spaces also generate complex environmental and social challenges that can only be met by active public interventions at multiple scales, at the household (e.g. housing subsidies), city (e.g. planning) and national levels (e.g. trade and employment policies). If these challenges arc not dealt with effectively, the welfare of individuals, communities and entire nations can suffer. These are no inevitable consequences of rapid urban population growth; rather, they are a consequence of political and institutional failure that inhibits effective urban planning, policymaking, investment and regulation.