In this book, Marc Morjé Howard addresses immigrant integration, one of the most critical challenges facing European countries today, the resolution of which will in large part depend on how foreigners can become citizens. Howard's research shows that despite remarkable convergence in their economic, judicial, and social policies, the countries of the European Union still maintain very different definitions of citizenship. Based on an innovative measure of national citizenship policies, the book accounts for both historical variation and contemporary change. Howard's historical explanation highlights the legacies of colonialism and early democratization, which unintentionally created relatively inclusive citizenship regimes. The contemporary analysis explores why some of the more restrictive countries have liberalized in recent decades, whereas others have not. Howard's argument focuses on the politics of citizenship, showing in particular how anti-immigrant public opinion - when activated politically, usually by far right movements or public referenda - can block the liberalizing tendencies of political elites. Overall, the book shows the far-reaching implications of this growing and volatile issue.