The preliminary ruling procedure in Article 267 of the Treaty on the Functioning of the EU is the most important procedure of EU law. There are, however, growing indications that this procedure is not working optimally: national courts seem to lack the necessary knowledge of EU law or they appear simply unwilling to refer. It is thus crucial to understand why and how national courts use the procedure. This PhD project therefore looks into the motives for referral/non-referral of judges of various courts in three EU Member States.