The genesis of the principle of legal certainty: from global standards to the doctrine of the Constitutional Court of Ukraine

The article comprehensively analyses the genesis of the principle of legal certainty as a fundamental component of the rule of law in the context of global legal traditions and the peculiarities of its implementation in the Ukrainian legal system. It is emphasised that although the rule of law is enshrined in the Constitution of Ukraine, the principle of legal certainty itself was formed through the evolutionary development of legal doctrine and the gradual expansion of its content in the practice of the Constitutional Court of Ukraine. Particular attention is paid to the Ukrainian legal tradition, primarily the Ruska Pravda, which already contained elements of orderliness, predictability and normative clarity that defined the limits of behaviour of participants in legal relations. The doctrinal approaches of the 19th century, which gave this principle conceptual completeness and laid the foundation for its further integration into international and Ukrainian legal thought, are also analysed. It is revealed that in modern Ukraine, the principle was established mainly through the decisions of the Constitutional Court of Ukraine. In its 1997 decision, the Constitutional Court of Ukraine defined legal certainty for the first time as a guarantee of the stability of the legal order and emphasised the prohibition of retroactive laws. In its 2001 decision, it was emphasised that the irreversibility of normative legal acts is the basis of citizens’ trust in the state and an important condition for legal stability. After 2022, the speed and frequency of changes in legislation, as well as violations or reductions in vacatio legis, brought to the fore the issues of legitimate expectations and the predictability of legal regulation. Particularly important in this regard is the decision of the Constitutional Court of Ukraine of 21 January 2025, which clearly recognised the need to establish an appropriate transition period as a condition for the implementation of legal certainty and the protection of legal subjects from legal uncertainty. It was concluded that the principle of legal certainty has undergone a long process of development, during which it has acquired its own substantive content. In the process of adaptation to the national legal order, this principle has been transformed into a key instrument for ensuring the stability and predictability of legal regulation, as well as an important mechanism for strengthening citizens’ trust in the state. Its integration into the practice of the Constitutional Court of Ukraine has given the principle not only theoretical but also real regulatory significance, transforming it into a fundamental condition for the effective and consistent functioning of the legal system.