Most countries around the globe have one or two levels of regional or intermediate government, yet we have little systematic idea of how much authority they wield, or how this has changed over time. This book measures and explains the formal authority of intermediate or regional government in 42 advanced democracies, including the 27 EU member states. It tracks regional authority on an annual basis from 1950 to 2006. The measure reveals wide variation both cross-sectionally and over time. The authors examine four influences -- functional pressures, democratization, European integration, and identity -- to explain regionalization over the past half-century. This unique and comprehensive volume will be a vital resource for students and scholars of comparative politics, public administration and public management, federalism, democratization, nationalism, and multilevel governance.