In the United States the density of economic activity in cities differed significantly over time. Between the eighteenth and early nineteenth centuries, cities became increasingly dense; however, since the second half of the twentieth century, the trend has reversed as cities have become dispersed. The density of economic activities differed significantly across cities as well. At any given point in time, there is a great variation in the density of economic activities across cities. The dynamic changes in the spatial organization of economic activities also differed across cities. In some cities, such as New York, population density grew and plateaued while in others, such as St. Louis, it grew and fell dramatically. Yet, despite the fact that the defining element of a city is its density, few scholars have systematically examined the long-run changes in the spatial organization of economic activities. This paper documents the historical changes in the density of economic activities and explores the causes of their rise and decline.