Stating that no one must be subject to criminal proceedings for an offence for which he has already been finally acquitted or convicted within the European Union, Article 54 of the Convention Implementing the Schengen Agreement and Article 50 of the eu Charter of Fundamental Rights have provided the ne bis in idem principle with a transnational effect. This protection from double criminal punishment throughout the Union crucially depends upon whether criminal proceedings in a Member State have resulted in a “final” decision (acquittal or conviction). According to well-established case-law of the Court of Justice of the European Union, the ne bis in idem principle does not require the judgment of a court, but may also be triggered by the decision of a public prosecutor (e.g. a transaction). The Court, however, has recently held that the requirement of a final acquittal or conviction is not met if the decision has not been given after a determination as to the merits of the case (judgment of 29 June 2016, C-486/14, Kossowski). This paper will argue that the notion of res iudicata should not be defined by Union law alone, but has to rely on national procedural law of the Member State where the decision has been adopted; the key criterion should be whether or not the decision is “final” under national law. Thereby, the transnational effect of the ne bis in idem principle depends upon the scope and limits of res iudicata under national law.