The aim of this article is to review the existing discipline of legal institutes in connection with significant violations of substantive or procedural legal norms and its significance in criminal proceedings in the Republic of Latvia and the Republic of Lithuania, their common and different features, problems and case law. The basis of the research is the analysis of regulatory enactments, court rulings, findings and opinions. The research is based on analytical and synthesis methods study of the correlations and differences in the legal regulation of criminal procedural law, comparative method comparison of specific legal regulations in the criminal procedural law of the Republic of Latvia and the Republic of Lithuania. The analytical met...