Because EU agencies are designed to facilitate the implementation of EU law at the national level, powers are increasingly conferred on them in order to ensure that rules are enforced effectively and uniformly. The time has come, however, to confront the many questions of legality and constitutionality that remain. This book responds to the vital as to the role and powers of agencies in relation to their manifold 'principals', the EU institutions and the Member States, and lays a firm foundation for managing the challenges ahead.