Two empirical regularities concerning the size distribution of cities have repeatedly been established: Zipf's law holds (the upper tail is Pareto), and city growth is proportionate. Census 2000 data are used covering the entire size distribution, not just the upper tail. The nontruncated distribution is shown to be lognormal, rather than Pareto. This provides a simple justification for the coexistence of proportionate growth and the resulting lognormal distribution. An equilibrium theory of local externalities that can explain the empirical size distribution of cities is proposed. The driving force is a random productivity process of local economies and the perfect mobility of workers.

We document the evolution of market power based on firm-level data for the U.S. economy since 1955. We measure both markups and profitability. In 1980, aggregate markups start to rise from 21% above marginal cost to 61% now. The increase is driven mainly by the upper tail of the markup distribution: the upper percentiles have increased sharply. Quite strikingly, the median is unchanged. In addition to the fattening upper tail of the markup distribution, there is reallocation of market share from low- to high-markup firms. This rise occurs mostly within industry. We also find an increase in the average profit rate from 1% to 8%. Although there is also an increase in overhead costs, the markup increase is in excess of overhead. We discuss the macroeconomic implications of an increase in average market power, which can account for a number of secular trends in the past four decades, most notably the declining labor and capital shares as well as the decrease in labor market dynamism.

We investigate the role of skill complementarities in production and mobility across cities. The nature of the complementarities determines the equilibrium skill distribution across cities. With extreme-skill complementarity, the skill distribution has thicker tails in large cities; with top-skill complementarity, there is first-order stochastic dominance. Using wage and housing price data, we find robust evidence of thick tails in large cities: large cities disproportionately attract both high-and lowskilled workers, while average skills are constant across city size. This pattern of spatial sorting is consistent with extreme-skill complementarity, where the productivity of high-skilled workers and of the providers of low-skilled services are mutually enhanced. If I can make it there I'll make it anywhere. ðFrank Sinatra, "New York, New York"Þ Rock Bottom, yeah I see you, all my Detroit people. ðEminem, "Welcome 2 Detroit"Þ We are grateful to numerous colleagues for detailed discussion and insightful comments. We also benefited from the feedback of many seminar audiences. Eeckhout gratefully acknowledges support by the European Research Council, grant 208068, and Schmidheiny