Extremist right-wing parties pose one of the most notable challenges to liberal democratic systems. This book breaks new ground by examining the organizational development of some of the most extreme parties in Europe at the local, rather than the national, level. The systematic analysis of evidence from hundreds of local branches and thousands of local activities demonstrates that even these highly centralized parties display notable subnational variation in how they develop. In some local settings, these really extreme parties have been particularly successful, and in others they have miserably failed. To account for the remarkable differences in local organizational life, this study explicates and then tests an analytical framework for understanding the local organizational development of extremist rightwing parties. It systematically examines how endogenous and environmental factors affect the capacity of these parties to penetrate local societies. This book builds on and pushes beyond the voluminous literature on the European far right parties and contributes to the broader literatures on political parties, social movements and militant democracies.