Since its establishment in 1951, the International Organization for Migration (IOM) has expanded from a small, regionally specific, logistically focused outfit into a major international organization involved in an almost dizzying array of activities related to human mobility. In 2016, IOM joined the UN system and has rebranded itself as the "UN migration agency." Despite its dramatic expansion and increasing influence, IOM remains understudied. This book provides an accessible, incisive introduction to IOM, focusing on its humanitarian activities and responses to forced migration - work that now makes up the majority of the organization's budget, staff, and field presence. IOM's humanitarian work is often overlooked or dismissed as a veil for its involvement in other activities that serve states' interests in restricting migration. In contrast, Bradley argues that understanding IOM's involvement in humanitarian action and work with displaced persons is pivotal to comprehending its evolution and contemporary significance. Examining tensions and controversies surrounding the agency's activities, including in the complex cases of Haiti and Libya, the book considers how IOM's structure, culture, and internal and external power struggles have shaped its behaviour. It demonstrates how IOM has grown by acting as an entrepreneur, cultivating autonomy and influence well beyond its limited formal mandate. The International Organization for Migration is essential reading for students and scholars of migration, humanitarianism, and international organizations.