The chapter introduces the theoretical approach of the book: The Advocacy Coalition Framework. It first discusses various important concepts of the framework. Thereafter, the chapter introduces the two main theoretical innovations of the study: The incorpotation of the notion of consensus into the framework and the importance of coalition resources. As consensus requirements are rather high in the EU, accounting for this institutional dimension is important to fruitfully apply the framework to the EU. Furthermore, as concerns the EU migration-development nexus, coalition resources are an extremely important asset for understanding this policy area.