The Danish accession to the Lisbon Treaty has been the center of a case brought before the Supreme Court. For the accession, the Danish government used the procedure in art. 19 of the Constitution instead of the more complex procedure in art. 20. In the case, it was argued that the accession to the Lisbon Treaty entailed a transfer of competences to the European Union and that art. 20 therefor should have been used. This raises the question of the constitutionality of the accession. This paper examines and discusses central elements of the Supreme Court’s decision. Significantly, structural changes, the identity principle and the validity of earlier accessions. In addition, the article examines how already transferred powers can be extended indirectly. In order to shed light on the complex issue, the article takes account of both national and international case law.