During the last decades, several West European states have gone through a process of decentralization, involving the shift of significant legislative powers to sub-national entities or regions. As a result their party systems have become multi-layered, featuring a variety of regional party systems that often differ considerably from the state-wide system. State-wide parties are the key integrative force in such a multi-level party system. Yet, in order to play their integrative role, state-wide parties must perform a difficult balancing act. Securing votes in regional elections requires that they give sufficient autonomy to their regional branches and give them some strategic leeway, without however jeopardizing the cohesion and programmatic coherence of the party as a whole. This book offers a powerful synthesis of research in party politics, federalism and regional politics and extends existing insights by bringing in new empirical evidence on the territorial organization and strategies of state-wide parties in five federal or regionalized states in Western Europe: Belgium, Germany, Italy, Spain and the United Kingdom.