How are processes of political development structured across space and time by preexisting institutions? This article develops a spatiotemporal theory of institutional change by analyzing the evolving infrastructural power of the European Union’s legal order using geospatial methods. Specifically, the authors theorize that the pattern and pace of the domestic spread of EU law has been shaped by preexisting state institutions—particularly by the degree to which national judiciaries are hierarchically organized. To assess this claim, the article compares patterns of domestic judicial enforcement of EU law across France (a unitary state with a centralized judiciary), Italy (a weaker unitary state with a centralized judiciary), and Germany (a federal state with a decentralized judiciary). Developing a geospatial approach to the study of legal integration and historical institutionalism more broadly, the authors leverage an original geocoded data set of cases referred to the EU’s European Court of Justice by national courts to visualize how the subnational penetration of Europe’s supranational legal order is conditioned by state institutions.