The nature of the Treaty provisions on EU external action, with a set of open-ended policy objectives and fewer policy-directed legal obligations on the Member States, has left much to the agenda-setting of the political institutions. The Court of Justice emphasises the need for the institutions to retain their discretion, their room for manoeuvre; it is non-interventionist, tending to take those choices at face value without seeking to define or shape them. Instead it has taken on another role: it ensures that the institutions act within their powers, and that the Member States do not obstruct the formation and implementation of Union policy. It is in fact engaged in establishing and protecting an institutional space within which policy may be formed, in which the different actors understand and work within their respective roles. The principles which have been drawn from the Treaties and elaborated by the Court to establish this institutional space are identified here as ‘structural principles’. They include the duty of sincere cooperation, the principles of conferral and institutional balance, mutual solidarity, subsidiarity, and the principle of autonomy. By identifying and developing these principles, which by their nature are flexible and capable of evolution, the Court of Justice exercises a formidable role in the governance of EU external action despite its hands-off approach to substantive policy choice. This paper seeks to explore further the nature of these structural principles as legal norms. It first offers an explanation for the importance of structural principles in the EU s external relations by exploring the nature of EU external relations powers. Second it begins an enquiry into the nature of structural principles: what does it mean to say that they are principles, that they are structural, and that they operate within external relations? Third, it offers a tentative typology of structural principles and some ideas on the ways in which they may complement and operate in tension with each other.