Just like cities themselves, urban economics is undergoing a period of resurgence. Yet urban economists have to date contributed very little to the development and evaluation of real-world urban policy. Both historically and currently urban policy has been much more influenced by the disciplines of engineering, architecture, design and planning than economics or other social sciences. To us this is both surprising and worrying. Cities are social and economic constructs-'organised complexity', as Jane Jacobs puts it in The Death and Life of Great American Cities (Jacobs 1961). As Ed Glaeser suggests, cities are probably our greatest invention. Other great inventions-the wheel or writing-arguably derive much of their value from the economic and social success of cities. Take writing: the complexity of urban life makes the efficient coding of information a necessity rather than a luxury, while the economic surplus created by cities in turn allowed the full cultural rewards of writing to be realised. Cities also amplified the impact of the wheel: to feed growing urban populations it was essential to overcome the 'tyranny of distance' (Bairoch 1988). Because of that success there were advantages in being able to support an urban population, and because they were more productive those early citizens demanded and could pay for better transport.