This paper quantifies the impact of large manufacturing plant openings on the total factor productivity (TFP) of existing plants in the same county, focusing on the spillover effects of agglomeration. The authors use the location rankings of profit-maximizing firms to compare plants in the winning county (where the new plant was located) with those in the runner-up county (the "losing county"). They find that five years after the new plant's opening, TFP in winning counties is 12% higher than in losing counties. This effect is more pronounced for plants that share similar labor and technology pools with the new plant. The study also finds evidence of increased skill-adjusted labor costs in winning counties, suggesting that the ultimate effect on profits is smaller than the direct increase in productivity. The paper discusses the theoretical framework, empirical predictions, and data sources, providing a robust analysis of the economic benefits and costs of agglomeration.This paper quantifies the impact of large manufacturing plant openings on the total factor productivity (TFP) of existing plants in the same county, focusing on the spillover effects of agglomeration. The authors use the location rankings of profit-maximizing firms to compare plants in the winning county (where the new plant was located) with those in the runner-up county (the "losing county"). They find that five years after the new plant's opening, TFP in winning counties is 12% higher than in losing counties. This effect is more pronounced for plants that share similar labor and technology pools with the new plant. The study also finds evidence of increased skill-adjusted labor costs in winning counties, suggesting that the ultimate effect on profits is smaller than the direct increase in productivity. The paper discusses the theoretical framework, empirical predictions, and data sources, providing a robust analysis of the economic benefits and costs of agglomeration.