This book offers a wide-ranging engagement with the work of Joseph Carens, one of the world's leading political theorists of the last four decades. Carens is widely credited with launching the philosophical study of immigration and the ethics of border control through his path-breaking scholarship, work that has also profoundly shaped our understanding of multiculturalism and citizenship. Moreover, he is a major figure in debates on distributive justice and egalitarianism, with G.A. Cohen describing his theory of markets and social justice as articulating "the platonic ideal of market socialism."This volume collects eight essays by world-renowned scholars discussing Carens's impact on these debates and on political theory more broadly. The essays not only clarify and challenge his contributions to political economy, multiculculturalism, citizenship, immigration, and border ethics, but also speak to the personal influence Carens has had on his colleagues, students, and interlocutors. The collection concludes with a response by Carens himself, offering his reflections on these interventions and the evolution of his thought.Justice without Frontiers will appeal to students and scholars in political theory, political philosophy, migration studies, citizenship studies, and multiculturalism. It is essential reading for researchers working on immigration ethics, distributive justice, and democratic theory, as well as anyone interested in the development of contemporary political thought.The chapters in this book were originally published as a special issue of Critical Review of International Social and Political Philosophy.