The author explores the place and significance of the constitutional complaint institution in implementing international standards related to life imprisonment in the national legal system. Recently, following the development of these standards in international human rights law, the European Court of Human Rights declared that the absence of a predictable and transparent mechanism for reducing life imprisonment in Ukraine violates human dignity and contradicts Article 3 of the European Convention on Human Rights. In alignment with this perspective, in 2020, the Constitutional Court of Ukraine deemed unconstitutional provisions that prohibited parole from life imprisonment. The constitutional complaint served as a tool for Ukrainian authorities to advance the life imprisonment reform. Apart from the constitutional complaint, no other mechanism has made such a powerful influence in this process. Consequently, it is necessary to analyze the constitutional proceedings that preceded the decision and the ratio decidendi within the same decision.The paper outlines aspects of the national model of the institution of life imprisonment and international standards governing this penalty. Discrepancies in these systems are identified, and the author investigates constitutional complaints filed in response to these disparities, along with the relevant constitutional proceedings. Subsequently, the author places significant emphasis on the decision of the Constitutional Court of Ukraine resulting from the consideration of the aforementioned complaints. In the author’s opinion, this decision acted as the driving force behind the continued reform of life imprisonment. The work also briefly outlines the prospects and challenges of life imprisonment reform, implemented even under martial law conditions.