This book theorizes a mechanism underlying regime-change waves, the deliberate efforts of diffusion entrepreneurs to spread a particular regime and regime-change model across state borders. Why do only certain states and non-state actors emerge as such entrepreneurs? Why, how, and how effectively do they support regime change abroad? To answer these questions, the book studies the entrepreneurs behind the third wave of democratization, with a focus on the new eastern European democracies - members of the European Union. The study finds that it is not the strongest democracies nor the democracies trying to ensure their survival in a neighborhood of non-democracies that become the most active diffusion entrepreneurs. It is, instead, the countries where the organizers of the domestic democratic transitions build strong solidarity movements supporting the spread of democracy abroad that do. The book also draws parallels between their activism abroad and their experiences with democratization and democracy assistance at home.