The study analyses the effects of the preliminary rulings rendered by the Court of Justice for the judicial body that made the reference and for other bodies dealing with similar cases, for the member states, for the European Union’ s institutions and for EU legal order. Starting from the binding effect of the preliminary judgment for national judicial bodies, which requires them to follow the ruling or make a new reference, to the lack of precedent doctrine in EU law, continuing with the possibility to indirectly verify the compatibility of national law of the member states with EU law and ending with the administrative or legislative measures that can or must be taken by the member states, the study intends to highlight the limits, nuances and consequences of the binding effect. It mentions the contribution of the national courts and of the Court of Justice of the European Union to the development of EU law, such as clarifying autonomous notions and it emphasizes the preliminary procedure's attributes of being a form of judicial protection of individual rights, as well as a means to review the legality of acts of EU institutions. The paper is meant to be a useful instrument for practitioners. Therefor, it also deals with the possibility and limits of asking new questions, in order to obtain reconsideration or a refinement of the legal issue and with the problem of judicial control over the interpretation and application of the preliminary ruling by the lower court.