Despite concerted efforts in recent years to define the position of agencies in the Union framework, a clear overall view of their role and powers in relation to the EU institutions and to the Member States is still lacking. Their hybrid character as part of the composite EU executive, and the fact that increasing powers are delegated to them, makes an understanding of the efficacy and accountability of agencies ever more important. Benefitting from both academic and practitioner insights from law, political and social sciences, this important book offers an in-depth analysis of the current challenges surrounding European agencies in terms of their design, autonomy, supervisory competence, and legal nature. Among the topics covered are the following: realities of the accountability mechanisms currently in place; impact of agency acts on the EU's institutional balance of powers; agencies as global actors acting on behalf of Member States and EU external relations; agencies derived from former networks of national regulators; non-hierarchical 'par' nature of agencies vis-à-vis corresponding national authorities; agencies as crucial amalgams between EU institutions and Member States; effect of the Meroni doctrine; new financial supervisory agencies resulting from recent economic and financial crises; special role of telecommunications agencies; and intricacies of the relationship between agencies and the European Parliament. 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.