"Urban proximity can reduce the costs of shipping goods and speed the flow of ideas. Improvements in communication technology might erode these advantages and allow people and firms to decentralize. However, improvements in transportation and communication technology can also increase the returns to new ideas, by allowing those ideas to be used throughout the world. This paper presents a model that illustrates these two rival effects that technological progress can have on cities. We then present some evidence suggesting that the model can help us to understand why the past thirty-five years have been kind to idea-producing places, like New York and Boston, and devastating to goods-producing cities, like Cleveland and Detroit"--National Bureau of Economic Research web site.