For a long time European law and criminal law have been considered – above all by national experts and academics – as completely separated legal areas. No reference to criminal law can be found in the Treaty of Rome, and because of that criminal law was not only considered to fall outside the competence of European institutions, but was even presented as being unaffected by European law and in general by European integration. The author shows this was not the case. In fact, since the very beginning, the dynamics of European integration have resulted in a significant impact on Member States’ discretion in deciding the exact borders and contents of national criminal law provisions; because, on the one hand, the non-selective nature of the principle of primacy of European law over domestic law (in the areas falling within the scope of EC competence, but that can integrate the contents of the criminal provisions), and, on the other hand, as a consequence of the general obligation on Member States arising out of the principle of loyal cooperation, which was progressively articulated (by EC secondary law and the ECJ) in very intrusive obligations on Member States when deciding on measures to be adopted to react to infringements to EC law (and then with the view to guarantee its effectiveness). In this perspective, the express conferral on the EU by the Treaty of Lisbon of a competence in criminal matters has definitively established the unequivocal integration of criminal law into the European legal order, and its evolution from a Member State exclusive competence into a shared one, subject (though with some important derogations) to the general principles supporting (proper) EU competences of this nature. The author goes through the main characteristics and forms of the competence in criminal matters conferred on the EU, dealing with the problematic issues arising both with the European law principles and criminal law principles, calling upon the crucial theme of sovereignty and democracy at EU level. She reaches the conclusion that, if well implemented, the conferral on the EU of competence in criminal law can ultimately not only benefit national States who have the opportunity to recover, through the EU, some part of the sovereignty they have lost in the global context, achieving by that a real democratic debate on issues that have escaped from a full democratic control at national level; but it will also improve the overall level of guarantees for individuals.