In this introduction to the book, the editors explain the relevance of analysing the constitutional aspects of the external dimension of EU migration and asylum policies. They argue that the labelling of the large arrival of refugees in 2015 as a crisis has severely affected the principles of the rule of law and the interinstitutional balance, which were just established with the Lisbon Treaty in 2009. The authors substantiate that the contributions in the book move beyond the state of the art in the literature by connecting the internal and external dimensions of EU migration and asylum policy and by analysing old and new patterns of external cooperation on migration. Through that lens, they identify a tendency of informalisation of the external cooperation, leading to ‘de-constitutionalisation’ of the EU decision-making in this field. This process raises questions on the EU’s legitimacy of the external cooperation on migration, which are dealt with in the book. The third part of the introductory chapter summarises the contributions in the book.