The chapter focuses on the interaction between European law and national criminal legislation, analysed in the light of the fundamental principles underpinning their relationship, especially the principle of primacy and the duty of loyal cooperation. After distinguishing between direct and indirect effects, paragraph 2 turns to the principle of primacy of EU Law. This cornerstone concept of Union Law affects domestic legislation in so far as it ‘neutralizes’ incompatible domestic criminal law through interpretation and disapplication of inconsistent provisions. This principle develops into a duty of loyalty for Member States (para. 3) which are thus bound to respect and enforce the rules and laws of the EU so as to secure the full and effective implementation of European criminal law in accordance with Art. 4 (3) TEU. Paragraph 4 examines how EU law penetrates the national legal systems of the Member States, focusing on the role of national judges that let European Law enter into the realm of national legal orders. This power represents the most relevant and disruptive manifestation of the ‘internal’ criminal effects of EU law. Selected case law will further demonstrate how EU law applies to and influences domestic criminal legal systems.