[en] The introduction to the collective book edited by the author analyses how the margin of appreciation of the national judge in the preliminary reference procedure depends on the double mandate to ensure effective implementation of European Union law and to guarantee an effective judicial protection. The national judge has a limited margin of appreciation while evaluating the need for a preliminary ruling and the relevance of the questions addressed to the Court of Justice of the European Union. The limitations stem from the consideration of the preliminary reference procedure as a tool to ensure effective judicial protection. The margin of appreciation of the national judge is more important while implementing EU law following the preliminary ruling of the Court, as he is is ensuring the balance between the Union’s and Member States’ interests.