The article analyses the core and contents of the proceedings based on newly discovered or exceptional circumstances (hereinafter – PBNDEC) in the criminal procedure law of Ukraine and other European countries. The authors emphasize that the PBNDEC are not designed to eliminate shortcomings of the pre-trial investigation and the trial in criminal proceedings, providing the reserve function in discovering and correcting court errors, but rather serves its own purpose in the mechanism of legal defence. The order the criminal procedure activity during this stage is largely determined by the grounds for its implementation – newly discovered or exceptional circumstances (hereinafter – NDEC). The differences between both groups of such circumstances are provided. Procedural features of the PBNDEC, which separate them from the proceedings on appeal and cassation, are singled out. The goals of this stage in the criminal proceedings are formulated, and the significance of this stage has been revealed. The need for improvement in the criminal procedure legislation of Ukraine which regulates the PBNDEC has been justified. The articles answer the questions of how efficient the regulation of the PBNDEC in the criminal procedure law of Ukraine is: whether the regulation is in agreement with the standards, whether it is established in the criminal procedure law of European countries, whether any doctrine positions regarding the core of such a stage in criminal proceedings require rethinking or additional theoretical justification, and which, of the normative regulations of such form of appeal and review of the court decisions, the principal directions for improvement are. In order to obtain answers to the aforementioned and other questions, the legal nature and the models of PBNDEC in the criminal legislation of European countries are considered in section two of the article. Section three depicts the procedural specifics of the PBNDEC in the criminal legislation of Ukraine. Section four describes the definition, shows the goals and significance, and introduces the structure of this stage in the criminal proceedings. Section five embraces the characteristics of features of NDEC in the criminal legislation of Ukraine. In section six the grounds for the PBNDEC in the criminal procedure law of Ukraine are analysed.