This issue, a joint editorial effort with Singapore’s Urban Redevelopment Authority, has a special focus on high-density cities. More than half of humanity lives in cities, and this proportion will keep growing. As urban densities rise, cities can chose to promote either higher density or sprawling growth. What are the pros and cons of density? To the extent that higher density is inevitable, how can cities be more liveable, sustainable and competitive? These questions preoccupy many of us, and we have tried to explore them here.