In Supranational Citizenship and the Challenge of Diversity Francesca Strumia explores the potential of European citizenship as a legal construct, and as a marker of group boundaries, for filtering internal and external diversities in the European Union. Adopting comparative federalism methodology, and drawing on insights from the international relations literature on the diffusion of norms, the author questions the impact of European citizenship on insider/outsider divides in the EU, as experienced by immigrants, set by member states and perceived by “native” citizens. The book proposes a novel argument about supranational citizenship as mutual recognition of belonging. This argument has important implications for the constitution of insider/outsider divides and for the reconciliation of multiple levels of diversity in the EU.