Abstract
As part of the European Convention, the principle of subsidiarity is marked by ambiguity. Originating from judges, it has not been formally defined in legal texts nor in any related jurisprudence and is therefore characterized by a semantic and legal indeterminacy. Featuring both a procedural and a tangible dimension, it remains a fundamental principle of European law. On the one hand, it structures, the system’s organization and operations. On the other hand, it guides the court of Strasbourg’s interpretation and review. Naturally ambiguous but still guiding European law, the principle of subsidiarity is presented as a flexible and malleable principle, and therefore adaptable. The analysis of its implementation in the European jurisprudence highlights its inconsistency and variability at the free disposal of the court of Strasbourg. However, in the light of the current context of an unprecedented saturation of the system and therefore of an urgent reassertion of its subsidiary nature, a fluctuating use of the principle is assessed differently to make it not a guarantee of national liberties, but a guide for them. As a driving force for increased accountability of member states, the principle of subsidiarity appears to be essential to the judicial policy of the European court.