The principle of ne bis in idem prohibits an individual from being tried or punished more than once for the same offence. This paper examines its historical roots, its codification in Italian law, and its development in supranational frameworks — notbly, Article 4 of Protocol No. 7 to the echr and Article 50 of the eu Charter of Fundamental Rights. Through the analysis of national and European case-law, including the Engel test and decisions such as Grande Stevens, Menci, and A and B v. Norway, the study highlights the conditions under which the coexistence of criminal and administrative proceedings can be accepted, and the necessary safeguards required by the proportionality principle. The paper closes with a critical reflection on future directions in national and European jurisprudence.