One of the most robust findings emerging from studies of industrial agglomeration is the rise in productivity that tends to accompany it. What most studies have not addressed, however, is the potential role played by human capital externalities in driving this relationship. This paper seeks to do so using data from the 1980, 1990, and 2000 US Census covering a collection of 77 (primarily) 3-digit manufacturing industries across a sample of more than 200 metropolitan areas. The analysis generates two primary results. First, a variety of education- and experience-based measures of average human capital rise significantly as an industry's employment in a metropolitan area increases. Hence, clusters of industry do tend to be characterized by larger stocks of human capital. However, second, even after accounting for the level of human capital in a worker's own industry, the overall size of the industry remains strongly associated with wages. Such results suggest that localization economies are largely not the product of knowledge spillovers.