The question what motivates judges to use the preliminary reference mechanism has been subject to academic debate for the past decades. More recently, a second wave of literature emerged which employs qualitative methodologies in an attempt to address this question. However, this empirical research only covers a few European legal systems and mostly focusses on the use of the preliminary reference mechanism by lower instance courts. Research on the reasons of highest national court judges to resort to the preliminary reference mechanism is however particularly interesting as many of the ‘grand theories’ on judges’ participation in the mechanism primarily see incentives for lower instance courts to use the procedure, whereas they would expect last instance courts to be rather reluctant in doing so. Moreover, an inquiry into the reasons of highest national court judges to participate in the preliminary mechanism allows for a reflection on the role that the obligation to refer laid down in 267 TFEU and the CILFIT caselaw plays. The present working paper therefore inquires into the reasons of Belgian highest national court judges and law clerks (not) to make preliminary references to the ECJ about questions on the interpretation or validity of EU law and the role of the obligation to refer following from article 267 TFEU and the CILFIT case law in that decision. To answer this question, the paper builds on legal doctrinal and quantitative insights as well as on novel qualitative data gained through interviews with 10 judges and law clerks of Belgian highest national courts.