Over the 19th and 20th centuries, the political map of Europe changed considerably. At the end of this period, there were remarkably contrasting tendencies concerning the functions and densities of borders. The borders inside the European Union lost their importance, whereas Central and Eastern Europe saw the birth of a multitude of new state borders. The long-term study of border regions, therefore, is a relevant subject for geographers, historians, social scientists, and political scientists. The main thesis of this text is that the rise of the modern nation-state reinforced the separating function of state borders by nationalizing the people on both sides of them. This process gained strength in the late 19th and early 20th centuries, and was challenged in the post-war era by processes of supra-national integration, globalization and the revolution in communication and transport, as the case studies from different parts of Europe in this book show.