Over the past two decades, denationalisation – the controversial practice of revoking citizenship from unwanted citizens – has re-entered Western law and politics with astonishing haste. In this book, Christian Prener traces this remarkable development in the United Kingdom, Denmark, France and the United States and offers a timely and critical examination of the legal, moral, and political acceptability of citizenship revocation in response to acts of misconduct or disloyalty. Through an exploration of contemporary practices, caselaw and theory, the book distils some of the hard questions posed by the Western revival of denationalisation within international human rights law, moral philosophy and political theory as it probes the lawfulness, efficacy, and political legitimacy of revoking citizenship in the 21st century.