The design and implementation of the external dimension of the EU migration policy requires a high degree of coordination among the different levels and actors involved in its development. The current division of external competences in this field calls for the participation of both the Union and its Member States in the setting of cooperation with third countries in the field of migration, leading to potential overlaps as it is particularly noticeable, for instance, in the negotiation of readmission agreements or the adoption of mobility partnerships. The involvement of various EU institutions in the design and execution of this external dimension also creates major challenges for inter-institutional coordination within the EU, such as those identified in the relationships between the Commission, the eeas, the Council and the European Parliament. The same is true with regard to the different Directorates-General and working structures within the Commission or the Council. The legal answer to the current flaws of coordination which may be perceived in the development of the EU external action on migration lies in the “duty of cooperation” which has been inferred by the ecj from the loyal cooperation principle enshrined in art. 4.3 and 13.2 teu. This contribution attempts to extract from the Court’s case law the scope and content of the precise obligations arising from this duty and, after identifying those instances in which a lack of coordination and cooperation can be perceived at the different levels of action in the external dimension of migration, to suggest ways of ensuring its respect with the aim of securing the unity of the international representation of the EU and the consistency and effectiveness of its external action.