In introducing the volume, J. Toby Reiner emphasizes the importance of Spheres of Justice as a social-democratic alternative to liberal and libertarian theories of justice. Walzer’s brand of social democracy has the potential to revitalize left-wing thought because its embrace of the “shared understandings” of specific societies means that it speaks to citizens in the language of everyday political debate. Walzer’s particularism thus offers an important corrective to the universalism of much Anglophone political theory, while retaining a critical edge by distinguishing deep values from contemporary practices. Spheres has also made a major contribution to political theory by directing the attention of scholars away from the foundations of philosophy and toward topics such as migration, healthcare, and cultural diversity that are of pressing concern to contemporary politics. Reiner surveys the major arguments and contribution of Spheres, sets the work in the intellectual context of Walzer’s editorship of New York Intellectual magazine Dissent, and provides an overview of the other chapters in this volume, and of the themes that unite them, above all the status of shared understandings in an age of political polarization.