Less than one month after the Spanish Constitutional Court required for the first time the European Court of Justice to give a preliminary ruling (article 267 TFEU), the Center for European Studies of IE Univesity, in collaboration with IE Law School, organized, under the direction of Daniel Sarmiento, the fourth seminar of the Global Judicial Forum specially devoted to this issue. Pedro Cruz Villalón, Advocate General of the European Court of Justice, Laurence Burgogue-Larsen, Professor of International Public Law of the Panthéon-Sorbonne University and Luis Arroyo, letrado at the Spanish Constitutional Court participated in the seminar. The discussion was very fruitful thanks to the three speakers as well as to the interventions during the debate of Judges of the Spanish Supreme Court, prosecutors from the Attorney General, State attorneys and academics.