The constitutionalisation of the European Union has since the early 1990s become a truism in European studies. This article revisits the constitutionalisation theory drawing on the insights from emerging historical research and new strands of political science research. We find that the conventional constitutional narrative is less convincing when confronted with the new evidence from historical and political science research. New historical research show that Member State governments, administrations and courts have generally been rather reluctant to embrace the constitutional project of the ECJ. Furthermore, at the level of European politics, the ECJ and its case law have far from judicialized European decision-making to the extent often claimed. Concluding, we reject the notion that the ECJ has successfully constitutionalised the EU, emphasising instead the inherent tensions in the process, which continue to complicate the efficiency of European law.