The thesis examines how EU courts have addressed the rule of law challenges of composite procedures. Composite procedures are pervasive administrative processes which involve joint decision-making by national and EU authorities. Such procedures fit poorly into the EU’s traditional model of administrative law, EU executive federalism, which is designed for an administrative system where decisional power is exercised separately by the two levels of administration. This mismatch would make it difficult to observe several key requirements of the rule of law in EU administrative law – such as the right to be heard, the right to a reasoned decision, judicial protection, and the control of legality. The thesis argues that EU courts have crafted a series of unprecedented implicit principles that specifically aim at ensuring the observance of rule of law requirements in composite decision-making. In doing so, EU case law has departed from the old doctrine of EU executive federalism. This was however not an easy transition. Indeed, since the EU’s foundational period, EU executive federalism was considered to be a constitutional doctrine, i.e., to immediately flow from the Treaties. Given the almost complete lack of references to administrative issues in the Treaties, this reading was entirely question-begging. Its espousal in the case law is explained in the dissertation as the likely result of a shared federalist conception of the European Union and of the administrative order created under its aegis. The thesis further argues that, just as the doctrine of EU executive federalism, the judge-made law of composite procedures relies on a series of assumptions on the relations between national and EU administration. The principles of composite decision-making do not treat national and EU authorities as two strictly separate spheres of power. Rather, they handle the two levels as a single, integrated administration, where national authorities are treated as an extension of the Commission – as the EU administration’s ancillary bureaucracy.