Within the EU’s judicial hierarchy, national courts can directly refer questions regarding the application and interpretation of EU law in member states to the European Court of Justice, creating a decentralized system of enforcing EU law. The Lisbon Treaty’s Article 267 specifies that national courts can refer cases to the ECJ to ‘specify the validity and interpretation of acts.’ Why do national courts make use of Article 267 submissions in some of the cases they hear but not in others? In her article “The decentralized enforcement of European law: national court decisions on EU directives with and without preliminary reference submissions” published in the Journal of European Public Policy, Carolin Hübner argues that national courts may simply follow the letter of the Treaty. Drawing on evidence from a sample of 1,310 national court decisions on EU directives, Carolin shows that cases involving directives that have left EU member states with more room for interpretation in implementation and technically complex directives are more likely to result in an Article 267 submission by national courts. The existing academic literature generally expects courts to use the preliminary reference procedure to alert the ECJ to non-compliant member states. Qualifying this expectation, Carolin’s analysis suggests that it “may well be the case that the mechanism is much more integrated into national courts’ day-to-day consideration of legal questions than the literature assumes.”