This book examines the European Court of Justice's principles relating to composite decision-making. Through rigorous case law analysis, it shows how these rely on national and Union observance of rule of law requirements, under what the book calls the 'Unitary Protection' doctrine. It explores the theoretical dimension of this doctrine, illustrating how it represents a departure from the EU's foundational federalist approach to administrative law. This fills a long-standing gap in the literature and in our full understanding of composite decision-making, a key tenet of EU law. EU constitutional and administrative law scholars will be fascinated by this compelling study.