The ne bis in idem principle (procedural defence) proscribes multiple criminal proceedings and punishments for the same criminal offence/conduct, which is predicated on a final verdict of acquittal or conviction by a court of competent jurisdiction. Incorporated as a fundamental human right through Article 4 of Protocol No. 7 annexed to the ECHR and fundamental right safeguarded through Article 50 CFR, the principle is conjoined with the right to free movement of persons through Article 54 CISA. An evaluation of the characteristics, substance, rationale, scope, and limitations associated with the autonomous procedural defence reveals corresponding purposes. CJEU and ECtHR jurisprudence have delineated the scope and limitations of the procedural defence and the two European courts have reciprocally influenced their respective case law. Definitive and practical judicial guidelines on the application of the principle facilitate consistency of approach by diverse national legal systems consistent with the principle of legality. The article provides an interpretative analysis of the procedural defence and associated jurisprudence of the two European courts that aim to ensure consistency of approach by national legal systems notwithstanding the applicability of the margin of appreciation and the principle of subsidiarity. The article concludes with an evaluation of the narrow same criminal offence criterion (idem crimen) commensurate with broader proscribed conduct (idem factum) criterion that is pivotal to the application of the procedural defence and evidently the most litigated aspect of the ne bis in idem principle. The main issue causing tensions between the different streams of case law seems to be the question of the combination of distinct types of proceedings (administrative and criminal), and there is a reciprocal influence of the ECHR and CJEU on this issue.