The article analyzes theoretical approaches to the understanding system of principles of environmental protection law in Ukraine and the Republic of Poland. The author interprets fundamental normative-legal acts and summarizes views of Ukrainian and Polish scientists regarding the understanding of fundamental basics of environmental-legal regulation of social relations. The system of principles of environmental protection law of Ukraine is traditionally derived from the normatively established list of principles in Art. 3 of the Law of Ukraine “On Environmental Protection”. At the same time, more generalized or, conversely, detailed lists of the principles of environmental protection law are presented in educational and scientific Ukrainian legal literature. The Polish fundamental law on environmental protection does not contain a single list of principles, but includes a number of separate articles, united under the category of general principles of environmental- legal regulation. In the Polish environmental-legal doctrine (as well as in the Ukrainian one) there is no single approach to the systematization of principles. The article states the main approaches presented in the scientific literature and the proposed system of principles in the doctrines of both states, which coincide in many positions. At the same time, the author notes the tendency of recent years, both among Ukrainian and Polish scientists, to a broader analysis of principles of environmental protection law, which are based on general principles of EU law, such as justice, proportionality, subsidiarity. In a narrow understanding of principles of environmental protection law, scientists emphasize the principle of sustainable development, prevention, an integrated approach, and ensuring public participation. In environmental protection law of Poland, the principle of integration is of particular importance, which implies the need to take into account environmental protection issues in various sectoral policies, strategies, plans. In Ukraine, the principle of the ecosystem approach is gaining actual importance and significant attention in works of scientists.