Non bis in idem principle in tax criminal law

The principle of non bis in idem is among the fundamental principles of criminal law in terms of protecting individual rights and ensuring legal security by providing the assurance of not being tried and punished more than once for the same act. This thesis analyses the applicability of this principle in the Turkish legal system in the context of tax criminal law and presents a comparative evaluation in line with the case law of the European Court of Human Rights. In the study, firstly, the historical development of the principle of non bis in idem and its place in international law are discussed, and then the elements of "same person", "same act" and "double jeopardy or punishment" are explained. Especially within the framework of the ECtHR's Zolotukhin and A and B v. Norway judgements, it has been determined that the "criminal nature" of administrative fines expands the scope of the principle. In Turkish tax criminal law, it is observed that tax loss penalties and tax offences under the Tax Procedure Law lead to parallel judicial processes in relation to the same act; this situation is considered to have problematic consequences in terms of the principles of legal security and fair trial. Recent decisions of the Constitutional Court draw attention to the need for normative harmonisation in this area. The study also analyses the legislative and implementation reforms carried out in the Scandinavian countries in the process of harmonisation with the principle of non bis in idem and reveals that the integrated coordination of administrative and criminal sanctions in these countries prevents duplication. As a result, this thesis presents structural proposals aiming to eliminate the gap between theory and practice regarding the principle of non bis in idem in Turkish tax criminal law; thus, it aims to contribute to both the effective protection of individual rights and the prevention of possible violation decisions.