Abstract
Result of a crossed-fertilization phenomenon, the principal of legal certainty has been enshrined since 1979 in the case Marckx v. Belgium. Rooted in the Rule of law, the principle within the meaning of Convention law is a multi-identity principle. On the one hand, on the other hand it is polysemous by the multitude of objective and subjective derivatives that compose it. Lastly, it is multifunctional in that it guarantees the stability and the reliability of the conventional system, by ensuring the protection of a plurality of interest of legal certainty: that of the States, that of the individuals and that of the conventional mechanism. Whereas its polyvocality confirms its notorious ambiguity, it is in reality symptomatic of a transmutation into a principle of legal “certainties”. If the promotion of a flexible conception of the legal certainty by the Court coincide with the effective protection of liberties and conventional rights, it deepens the legal insecurity by interfering with harming the effectiveness of rights. Furthermore, the unpredictability of its jurisprudence fuel the legitimacy crisis that is besetting its courtroom, which reduces the effectiveness of the law. The resorption of the legal insecurity lies within the research for palliatives to perpetuate the conciliation between the legal securities and the objective of effectiveness of the rights and of the law of the Convention.