How and to what degree do federations produce uniform law within their system?  This comparative empirical study addresses these questions comprehensively for the first time.  Originally produced under the auspices of the International Academy of Comparative Law, this volume examines legal unification in twenty federations around the world.  Each of the successive chapters presents the forces of unification through the lens of a particular federal system.  A comparative overview chapter provides a detailed analysis of the overall results with compelling visual illustrations of legal unification along different dimensions (e.g. by area of law; by federation; by civil vs common law system).  The overview chapter summarizes and analyzes the means and methods of legal unification and the degree of legal unification of each system, and explains the driving forces of legal unity and diversity in federations more generally.  The volume presents surprising findings that should make scholars rethink their abandonment of the civil law vs. common law distinction in comparative law. ​ This book is a milestone in the study of federalism. It is a rare and welcome melding of comparative law and comparative politics using both original data and qualitative analysis. Wide-ranging, probing, and definitive, this book is an invaluable resource for students of law, politics, and multi-level governance. Gary Marks, Burton Craige Professor, UNC-Chapel Hill, and Chair in Multilevel Governance, Vrije Universiteit Amsterdam