The idea of constitutional identity has many meanings and has been used in a variety of ways.[1] In Europe, the understanding of the term is of practical significance in particular because Article 4(2) of the Treaty on European Union (TEU), which requires EU law to respect national identities ‘inherent in their fundamental structures, political and constitutional’. Whereas Member States are generally under a duty to apply EU law even when it is in tension with national constitutional commitments, this provision has been interpreted to authorise Member States not to apply EU law, insofar they can successfully claim that the application of EU law would be incompatible with their national constitutional identity. In the following, I will refer to ‘national identity inherent in their fundamental structures, political and constitutional’ simply as ‘constitutional identity’.[2] In the context of Article 4(2) TEU, questions relating to constitutional identity thus become questions about 174the domain over which Member States can claim to be exempt from the duty to apply EU law. What is clear and uncontroversial is that national identities inherent in Member States’ fundamental structures, political and constitutional, are not already in play anytime a national constitutional provision, as interpreted by a national apex court, is in tension with the requirements of EU law. Article 4(2) TEU does not mean that the primacy of EU law applies only to national sub-constitutional norms and that constitutional norms generally prevail over EU law. Only a qualified set of norms and understandings that connects to Member States’ political and constitutional ‘fundamental structure’ allows Member States to claim exemption from the application of EU law incompatible with them. The question is what that means: what kind of norms can make up the constitutional identity of a Member State and thus provide the grounds for exempting that state from applying EU law when it is in conflict with them?