In the paper, we tried to comprehensively define the field of application of the prohibition of reformatio in peius, and to present theoretical, divergent, interpretations of this provision, as well as to try to illustrate all this with the practice of both regular and constitutional courts. We have seen that the provision of Article 400 of the Montenegrin Code of Criminal Procedure is not enough to be interpreted only linguistically. This provision hides much more. First of all, the phrase "appeal filed only in favor of the defendant" should be interpreted in the same way when the prosecutor filed an appeal that was rejected. In addition, we have seen that this prohibition binds both the second-instance court when deciding on the appeal and the first-instance court in the retrial, if the decision is revoked. The practice of international courts, more specifically the European Court of Human Rights and the Court of Justice of the European Union, represented an unavoidable sequence in the presentation. We have seen that the ECtHR did not affirm this prohibition in its practice. Moreover, it could be said that if the national courts respect the ECHR standards inaugurated so far, a stricter legal qualification and (or) a stricter criminal sanction against the defendant in the new proceedings would be allowed. It only remains for us to see whether the ECtHR, over time, will change this practice, or whether we, under the undoubted authority of this court, will marginalize this provision.