The aim of this work is two-fold. First it attempts to construct a more solid critique of the European Union principle of subsidiarity. Second it tries to explain why the European Court of Justice is not fully implementing subsidiarity. Introduced in the EU legal order as a last resortprotection mechanism for member states in a minoroty position in the Council of Ministers regarding a particular issue , the principle of subsidiarity is not only misplaced to fulfill that goal, but also its underlying logic has potential side-effects for European integration. Furthermore it isargued that, the 'legislation' of the principle in the Maastricht Treaty, and its reinforcement through the Amsterdam Protocol on subsidiarity cannot overcome the legitimacy dilemmas that the European Court of Justice would have to confront if it opted for the implementation of the principle. Thisbook gives an in-depth analysis of both issues, in the context of a wider analysis that concerns the legitimacy of the political and legal structures of the European Union.