This book offers an in-depth exploration of the European Union's cooperation with non-EU countries on readmission and return, as central components of EU migration policy. It delves into the differences between formal and informal agreements and the factors influencing return rates, critically assessing the 'effectiveness' of cooperation in this contentious policy field. Through a comparative analysis of 57 non-EU countries, the book enhances our understanding of why certain countries cooperate on readmission and/or return. Countries geographically closer to the EU are more dependent and, thus, conclude readmission agreements more often. For countries in Africa and Asia, EU incentives and pressure play a bigger role and influence the cooperation together with domestic administrative capacities and bilateral readmission agreements and closer existing relations. Formal or informal readmission cooperation has a limited impact on return rates with non-EU countries beyond the Eastern neighbourhood. This comprehensive work is an essential reading for academics, policymakers, and policy advocates interested in EU external migration cooperation and the broader dynamics of international migration policy.