The thesis describes shortly the activity of the Court of Justice of the European Union, the activity of the European Court of Justice and also the preliminary ruling procedure. It also comprises the twelve preliminary ruling request initiated by the Romanian High Court of Cassation and Justice from the European Court of Justice (Înalta Curte de Casaţie şi Justiţie) from the adherence of Romania to the European Union until 31st of May 2018. It needs to be emphasise that only closed cases were described and analysed in the present thesis from the matter of facts to the final decision of the national court