Economic activity is highly unevenly distributed across space. Understanding what drives the agglomeration and dispersion is important for many economic and policy questions. This column describes a theoretical model of internal city structure incorporating agglomeration and dispersion and heterogeneity in local fundamentals. The authors use the division and reunification of Berlin as a natural experiment. Their findings show that both heterogeneity in locational fundamentals and agglomeration forces are important in shaping a city’s internal structure.