This study highlights the principle of the legality of incrimination and penalty, as well as the principle of the retroactivity of the more lenient criminal law, as stipulated in Art. 7 of the European Convention on Human Rights. In order to clarify and highlight these principles, some cases of the practice of the Convention on Human Rights are presented. Furthermore, the material shows the way these fundamental principles are reflected in the Romanian legislation, namely the Constitution of Romania, the Criminal Code in force and the stipulations of the new Criminal Code. Understanding these principles at an European normative level and in the Romanian penal legislation, in order to apply them correctly in jurisdictional activity, is of particular importance in the current period of reform and adaption of the Romanian Criminal Legislation.