Theories of urban development stress the impact of knowledge spillovers in generating the positive externalities necessary for growth. The mechanism for generating these externalities, however, is a source of contention. They can arise, as articulated by Romer among others, from efficiencies derived by concentration of industry; from industry diversity due to inter-industry spillovers, as proposed by Jacobs; or from some interaction between the two. This study utilizes both fixed-effects ordinary least squares and differenced general method of movements models to determine the effect of concentration in specific industries as well as composition of the city as a whole on future population growth. Using a data set that includes industry-specific employment and demographic data for 313 metropolitan areas in the United States from 1969 to 2008, this study finds strong evidence that important knowledge spillovers occur between industries and mixed evidence for the existence of meaningful externalities within industries.