How do Muslim citizens across the globe perceive the European Union? And what factors influence their EU attitudes? This book offers the first systematic theoretical and empirical analysis of Muslim citizens’ EU attitudes in and outside the European Union. Using the best empirical data available, the book demonstrates that Muslim citizens’ attitudes are not shaped by their denomination and religious beliefs, but by material and political considerations. It finds that Muslims are most favourable toward the EU due to their positive experiences in European contexts, whereas in contrast, Muslim citizens outside the EU are more skeptical toward the European Union due to sovereignty concerns and the lack of support from the EU and its member states. Such findings not only contribute to the research on social legitimacy of international organizations and international public opinion more generally, but also provide important suggestions for (European) policy makers regarding external and domestic policies. This book will be of key interest to scholars, students and practitioners of European Union politics, Middle East studies, public opinion and International Relations.