The chapter deals with the constitutional aspects of Lithuania’s accession to the European Union, including the referendum on the accession, held in 2003, and the constitutional amendments aimed at enabling the accession. It is demonstrated that the Constitutional Court had a significant role to play in the process, both before and after the accession. A special emphasis is put on the EU law-friendly interpretation in the Constitutional Court’s case-law of the relationship between the EU law and Lithuanian law, and the rule of collision as formulated by the Constitutional Court in 2006. This rule consolidates the Constitution’s supremacy vis-à-vis EU law, but at the same time postulates the primacy of application of EU law vis-à-vis all other domestic legal acts. It is shown that the Court itself departed from this rule in 2017, and then reverted to it in 2020.