This chapter explores the constitutional dimensions of Lithuania’s membership in the European Union (EU), emphasizing that attention to EU law is not merely ceremonial, but reflects deep theoretical and practical legal engagement. Membership takes on new significance in the broader context of the EU’s efficiency, integration prospects, and responses to contemporary challenges—such as security, climate, and technology. The relationship between EU law and national law, particularly in Lithuania, has evolved beyond interest to a complex and nuanced field of daily legal practice. Debates in legal scholarship and at academic forums, including interpretations of the openness of the Constitution of the Republic of Lithuania to EU and international law, reveal a dynamic discourse. Notably, scholars such as T. Birmontienė question whether openness to EU law constitutes a principle of constitutional interpretation, while others emphasize the idea of EU law priority rather than supremacy, pointing to shared or transferred competencies. This chapter identifies and discusses four key constitutional aspects that, in the author’s view, reveal how Lithuania’s EU membership functions as both a constitutional value and a legal obligation, as interpreted by the Constitutional Court.