Thesis: This PhD dissertation studies how agglomeration economies shape the patterns of spatial concentration in French industries, and impact on French firm productivity. In the first chapter, we develop a new methodology to assess spatial concentration with micro-geographic data. This methodology is then applied to compare localization patterns in French service and manufacturing industries. In particular, we find that service industries tend to be more localized than manufacturing ones. In the second and third chapters, we assess the magnitude of urbanization and localization economies on French firm productivity. Chapter 2 proves that, on average, firms benefit from a larger density of employment in their vicinity and a more specialized environment. Chapter 3 considers the differential impact of agglomeration economies across heterogeneous producers. We emphasize that urbanization economies benefit more the most productive firms. Chapter 4 focuses on international trade issues. We find that the larger the stock of immigrants in a specific French département, the larger its trade flows toward the immigrants' country of origin. Finally, chapter 5 makes a methodological point by considering whether and how the choice of a specific zoning system, with spatial units of given size and shape, impacts on the statistical exercises of the previous chapters. We find that distortions due to the Modifiable Areal Unit Problem are of secondary concern in comparison with problems due to misspecification.