Since the beginning of the twentieth century, governments around the world have expelled, en masse, more than 30 million people. Yet despite its prevalence, mass expulsion remains an understudied phenomenon as scholarly attention has been directed toward the gravest atrocities--genocide and mass killing--or toward forced migration more broadly. Mass Expulsion: The Politics of Forced Population Removal examines why and how governments expel, providing a systematic, cross-regional account of mass expulsion over the longue durée. Using the original Government-Sponsored Mass Expulsion (GSME) dataset, the book explains when and where expulsion occurs, who is targeted, and what regimes are most likely to expel. The book identifies two overarching categories of mass expulsion--security and economic--and develops a typology of four corresponding types: counter-irredentist, counter-subversive, reprisal, and nativist. It advances a theory of mass expulsion that traces the decision-making process, showing how key structural, proximate, precipitating, and intervening factors combine to shape the conditions under which governments expel or are restrained. Evidence is drawn from archival research conducted at the United Nations High Commissioner for Refugees, the International Committee of the Red Cross, and the League of Nations archives in Geneva, Switzerland. This research contributes to the fields of international peace and security, political violence, and forced migration. Conceptually, it fills a gap in the literature by systematically examining mass expulsion as a distinct eliminationist policy aimed at the intentional removal of ethnic groups. Theoretically, it extends existing explanations beyond war and security threats, highlighting an entire class of expulsions targeting perceived economic threats. The book deepens our understanding of critical atrocity restraints, demonstrating the importance of alliances, the “homeland” state of the target group, and international organizations. These restraints generate core policy recommendations that seek to spur greater attention to deterring mass expulsion as a priority for atrocity prevention.