We investigate the factors influencing the choice of domestic judges to pass on cases to the Court of Justice of the European Union. While EU judicial scholars have typically relied on integration theory or ad hoc theories of adjudication to explain referral activity, our theoretical framework draws on general judicial decision making theory and focuses on the incentives and constraints national judges face when deciding whether to submit a references to the Court of Justice. Consistent with our rationalist account of judging, we find empirical evidence that the rate at which national courts refer cases is influenced by familiarity with EU law, power-seeking motives and political fragmentation.