Book excerpt: The study of how party systems are structured across territorial lines is a crucial research question for political scientists, whose answer is fraught with consequences for the political system and the democratic process. This book addresses this topic and raises the following questions. What has been the evolution of the vote nationalization process in Western Europe during the last fifty years? Which factors can account for the vote nationalization's variance across Western European party system? Through a macro-comparative perspective and an original empirical research, involving 230 parliamentary elections occurred in sixteen countries during the 1965-2015 period, this book provides answers to these questions. It analyses the evolution of vote nationalization in Western European party systems over the last fifty years and looks for an explanation. The result is a far-reaching understanding of the macro-constellation of factors involved in the process, including macro-sociological, institutional, and competition determinants.