This paper discusses the concept of Europeanization which became very popular in research focused on the national adaptation of EU member-states to developments on the supranational level. The paper highlights the theoretical roots of the Europeanization studies, especially multi-level governance and new institutional theories. The author points out the definitional ambiguity of Europeanization concept. Its «stretching» on other phenomena is its main weaknesses. Many definitions confuse the processes of Europeanization and European integration itself. The author argues that the explanation of Europeanization should be based on systems theory. It provides a methodological basis for studying the processes of transformation both on the national and supranational levels.