In Chapter VI, the fragments of the legality principle as it is currently conceived and applied in European criminal law were discussed. Certain questions, inconsistencies and lacunae were identified. In Chapter VII, the legitimacy of the European criminal justice system was discussed and a relational model was defended. How should the principle of legality function in a relational model of European criminal justice? In this final Chapter, a coherent doctrine of the legality principle for European criminal law will be presented. This Chapter is structured as follows.

In the first section, the values supported by the legality principle are addressed. Most of the theoretical foundations of the European criminal justice system were presented in the previous Chapter. The rationales shall be further concretised as they relate to the legality principle.

The second section will explore the nature and scope of the legality principle within European criminal law through two themes. The first theme addresses the role of the legality principle in legitimising the exercise of criminal law competences at the European level. In national law, the legislator is sovereign and decides what behaviours are criminal; however, this is not the case in European criminal law. Whether the legality principle should play a role in determining competences and how to justify this practice is also explored.

Another problem to be assessed is the application of the legality principle in a two-level system. Is this principle to be addressed to the European or national authorities, or both? As seen in Chapter VI, it is difficult to understand how lex certa, for example, would apply to Directives. Therefore, the distributive role of the legality principle shall be addressed.

The third section will delve deeper into the normative role of the legality principle, namely specific applications of the legality principle. Certain questions raised in Chapter VI are revisited here. For example, should case law be a formal source of criminal liability? If so, one would have to explain the features of each aspect of the legality principle.

General conclusions are presented in the fourth and final section, along with a recap of the initial research question and reflections on how it has been answered herein. The section concludes with further remarks, suggestions and questions for the future.