In an attempt to strengthen the legitimacy of European Union (EU) policy-making, the 2009 Lisbon Treaty strengthened the principle of parliamentary control in EU affairs. This pertains to parliaments at all levels ranging from the regional to the supranational level. This book analyses the potential of regional parliaments - the parliaments closest to the citizens - to engage in EU affairs and to fill the perceived legitimacy gap. Eight member states have a total of 73 regional parliaments with legislative powers, and there are further trends towards decentralization in Europe. On its quest to understand the role that regional parliaments can plan in the EU multi-level parliamentary system, the book addresses key questions: What are the formal powers and functions of regional parliaments in EU policy-making? How do they use their powers in practice? How active are they in EU politics, and what do they try to achieve? What factors can explain their degree of (in-)activity? The chapters in this book were originally published in a special issue of The Journal of Legislative Studies.