The establishment of European Union criminal law entails the novel situation in which EU actors influence the definition and interpretation of domestic crimes and penalties. Both the Union legislature and the CJEU define and interpret provisions of EU law with relevance for the determination of criminal liability and the prescription of the applicable penalties in the law of the Member States. This influence on substantive criminal law raises questions about the limits to these legislative and interpretive activities, both at the EU level and at the level of the Member States. Since requirements for the definition, interpretation, and application of substantive criminal law are traditionally provided by the principle 'nullum crimen, nulla poena sine lege' (ie the legality principle), the functioning of this principle in the legal field of EU criminal law merits investigation. With the aim of comprehending the role and functioning of the legality principle in EU criminal law, this thesis examines and compares the actual constructions of the supranational European legality principles; ie the legality principles protected under the ECHR and in EU law. The present research first determines that the ECHR legality principle requires the protection of only a rather minimal standard of legal certainty. While the protection of such a minimum standard could be appropriate under the ECHR, this is not necessarily the case for EU law. Due to the multilevel nature of the definition and interpretation of offences and penalties in EU criminal law, the influence of multilingualism, and the general Union standard of legal certainty provided outside the criminal sphere, it is appropriate for the EU legality principle to move beyond the minimum ECHR standard. Instead of functioning as a prohibition on arbitrariness, it is argued, the EU legality principle should ensure a minimum level of legal certainty that is closer to the maximum predictability of consequences for certain acts. It is additionally argued that the EU legality principle could be construed more consistently and on the basis of a clear conceptual framework, and that the principle’s general conformity with the ECHR minimum standard could be made more apparent.