This book studies both the causes and effects of country specialization and industry concentration. First, a detailed literature review of both theoretical and empirical work is given. Second, the most important indices used in studying the heterogeneity of countries and industries are analyzed. The book discusses the (dis-)advantages of these indices in detail and shows how empirical results depend on the index chosen. Third, the book studies the economic development of individual industries in detail and identifies crucial structural breaks in the development paths. Last, the heterogeneity across Western European countries is analyzed and the countries are assigned to individual clubs. In doing so, this book examines the similarity of countries in their economic development and identifies the drivers of these processes.