Abstract
Since the 2000 Lisbon European Council, the European Union has worked to complete its transition to a knowledge-based society. In this context of permanent technical and technological evolution, judges have the option of calling on experts to help them understand the increasing complexity of the cases brought before them. However, an expert’s opinion must not be taken for a judge’s ruling. Furthermore, in addition to procedural guarantees regarding the selection of experts, the Court of Justice of the European Union has defined criteria of legitimacy in order to guarantee the principle of res judicata. It is thus important to analyze how the Court reconciles its institutional obligation to guarantee the value of its rulings with the legal and economic necessity of protecting the technical value of expert opinions. This analysis may be broadened to incorporate issues relating to administrative decisions.