The fragmentary application of the principle of legality in European criminal law is a consequence of deeper deficiencies regarding the system's legitimacy. The attribution of criminal law competences to European institutions has been determined to be mainly a reflex to the negative consequences of abolishing internal borders, although an incremental shift towards a more principled criminal justice has been noted. Currently, principles such as ultima ratio, proportionality, legality and guilt are considered to govern European criminal law, at least in theory. However, the theoretical foundations of the European ius puniendi and what these principles represent remain unclear. Does European criminal law offer a form of protection to individuals against arbitrariness, and if so, what would be the nature of such protection? Questions about the instrumental finality are also in order. What is the rationale of combating cross-border crime or facilitating judicial cooperation? Often, boosting freedom of movement has been cited as the raison d’être, but just how far can this argument go? In principle, every harmonisation of criminal offences would serve this purpose.

This Chapter will discuss the legitimacy of European criminal law. As with national criminal law, the function of the legality principle in EU law depends on how criminal law is legitimised. It was explained in Chapter I that the legality principle has a legitimising function. Criminal law legitimacy was defined as the justification of coercive power. To construct a theory for this principle in EU law, it is important to understand what type of legitimacy it should provide. However, the question of legitimacy is quite complex. The European criminal justice system presents various unprecedented particularities that create a matrix of interrelated problems.

First, in national law, the main focus of the principle of legality is the relationship between individuals and the state. But the relationship between the EU and individuals in the field of criminal law is not clear. The purpose and nature of the EU in the field of criminal law is also uncertain. European criminal justice forms a part of the European legal order to which its purpose and nature are strongly connected.