This paper examines the prevalence of localized manufacturing industries in the United States, particularly focusing on the Silicon Valley and Detroit examples. The authors use a "dartboard approach" to model firms' location choices, considering both spillovers and natural advantages as factors influencing agglomeration. They develop an index of geographic concentration to measure the degree of localization, controlling for plant size distribution and geographic area size. The index is shown to be unbiased and allows for meaningful comparisons across industries. The study finds that almost all industries exhibit some degree of localization, though in many cases, the degree is slight. The authors explore the nature of agglomeration forces, the geographic scope of localization, and the extent to which agglomerations involve plants in similar or identical industries. The paper also discusses the methodology and provides a detailed description of geographic concentration patterns in U.S. manufacturing industries.This paper examines the prevalence of localized manufacturing industries in the United States, particularly focusing on the Silicon Valley and Detroit examples. The authors use a "dartboard approach" to model firms' location choices, considering both spillovers and natural advantages as factors influencing agglomeration. They develop an index of geographic concentration to measure the degree of localization, controlling for plant size distribution and geographic area size. The index is shown to be unbiased and allows for meaningful comparisons across industries. The study finds that almost all industries exhibit some degree of localization, though in many cases, the degree is slight. The authors explore the nature of agglomeration forces, the geographic scope of localization, and the extent to which agglomerations involve plants in similar or identical industries. The paper also discusses the methodology and provides a detailed description of geographic concentration patterns in U.S. manufacturing industries.