The idea of flexibility seems to provide an attractive European model to aim for, not least to avoid the accusation of a ‘one-size’ fits all approach to the highly complicated project of an EU of 28 Member States. Yet, as this chapter will try to show, as much as the notion of flexibility seems to be an attractive concept, it is also a rather vague one. Specifically, this chapter sets out to explore the notion of ‘flexible’ integration in the concrete area of EU criminal law. The phenomenon of EU criminal law is traditionally meant to capture cross border criminality where the EU is taking multilateral Member State action to combat particularly serious criminality such as terrorism, organized crime and cyber-crime related criminality. However, in recent years, the EU has acquired a more freestanding competence to legislate on criminal law proper.