The development of the EU principles, particularly of the subsidiarity principle and of the proportionality principle, follows different directions, according to the different legal systems of Member States. In Germany, where the principles took origin, the structure of the Constitution itself has been deeply transformed. In Italy, even if less strongly than in German system, the devolution of powers from the State to the Regions is inspired to the same criteria. The Constitutional Courts of both the Member States are becoming familiar with these principles as well, in order to satisfy the needs of their effectiveness. The possibility of a judicial review, technical and impartial, may be used as a fundamental tool to trespass the threshold of “discretionary power” and to make the subsidiarity principle more enforceable.