The present academic contribution does not start from scratch. A European legality principle already exists, albeit a fragmentary one. This principle did not emerge from a vacuum but from the national legal traditions. This research shall therefore follow the methodology with which the ECJ has utilised the principle of legality from national systems and adapted it to the EU context. In particular, the principle of legality in EU law has a twofold status. It is recognised as a general principle of EU law, and more recently, it has been codified as a fundamental right in Article 49 of the Charter. As a general principle of EU law, the principle of legality was developed through ECJ judgements throughout the 1980s. General principles of law exist in most Western legal systems and are sources of law filling gaps of legislation. This can happen through a specific reference to legal texts (such as Article 7 para. 2 ECHR which refers to the general principles of law recognised in civilised nations) or spontaneously by courts. Here one should be careful with terminology and distinguish ‘general principles of EU law’ from the ‘general principles of law’ in a specific legal system or international law (as in Article 7 para. 2 ECHR). The former represent specific documented principles developed by the ECJ. General principles of EU law have a tripartite function: they are an aid to the interpretation of EU law, grounds for review of European and national implementing legal instruments, and their breach can establish a claim for damages.

The status of the legality principle as a general principle of EU law is essential for the methodological approach of this research. The general principles of EU law have a dual origin: they stem from common constitutional traditions of the Member States and international conventions, the primary influence being the ECHR. The ECJ usually performs a comparative analysis in order to identify whether a principle is accepted within Member States’ national legal orders and examines the ECHR as well. However, when these principles are recognised at the EU level and are adopted as part of EU law, they are instilled with a uniform and autonomous interpretation that may differ from that used in national legal orders; hence the principles are adapted and integrated into the EU's legal system.