The derivative concept of legal interpretation of Zygmunt Ziembiński and Maciej Zieliński is one of the two normative concepts of legal interpretation, generated in the Polish post-war theory of law; the second one is the semantic intentional (clarification) concept, formulated by Jerzy Wróblewski. According to the doctrine, the legal interpretation made by the Court of Justice of EU should mirror the wide, validation-derivation approach to the legal interpretation. This is due to the specific character of the sources of the EU law, and due to the evident, in this case, validation element, which excludes the case of isomorphy. The considerations concerning validation issues, taking into account the versatility of the sources of EU law, are very valuable, especially in view of the fact that the sources of the EU law differ in large degree from the sources of the national law. Accepting the derivative con-cept of legal interpretation in the EU context, implies the rejection of the doctrine of acte clair which seems useful from the point of view of the development of the co-operation of the national courts with the CJEU. The application of the derivative approach seems irreconcilable with the doctrine of acte clair. On the other hand, the interpretation practice of the EU law, connected with the multiplicity of the bodies, who have played the legislation functions, the obligation for making pro-Community interpretations (friendly and uniform), the spread of technology, regarding the majority of the EU legal regulations, all create real challenges for the derivative concept.