This chapter analyses IOM’s practices and policies on immigration detention from the 1990s to date, spanning a period of significant change in its approaches to detention. The chapter first distills pertinent international human rights law (IHRL) on migration-related detention, and then examines IOM’s normative statements concerning detention. It shows that while IOM generally emphasises international legal standards, it also tends to stress states’ ‘prerogative’ to detain, frame alternatives to detention (ATDs) as a desirable option rather than a legal obligation, and weave an operational role for itself, notably through assisted voluntary returns (AVRs). The chapter then interrogates IOM’s involvement in detention through four case studies. These reveal not only IOM’s changing role regarding detention, but its enduring part in a global system whereby powerful states and regions seek to contain protection seekers ‘elsewhere.’ The chapter concludes that, without constitutional and institutional change to ensure it meets its positive human rights obligations, and deeper critical reflection on its humanitarian duties, IOM’s practice risks expanding and legitimating detention.