This paper applies a consistent framework to four comparably sized metropolitan areas to identify and characterize their employment centers. Employment centers are identified as places that exceed a threshold employment density and a threshold employment level. They are also characterized as specializing on the basis of location quotient analysis. Clear evidence was found of specialization in every employment center in the four metropolitan areas studied. The interpretation is that what is observed is a systematic change in metropolitan structure rather than a random sprawling of firms. Evidence was also found that the size distribution of employment centers follows the rank-size rule. This suggests that there is structure not only in the distribution of economic activity among the employment centers but also in their size distribution.