The Intergovernmental Conference is a legal instrument for negotiating founding and amending treaties in the European Union. The monograph has four research objectives. The first is to reconstruct the dynamics of the Treaty changes in 1950-2024 and to present the prospects for EU Treaty reform after Russia's aggression against Ukraine. The second is to identify which Member States had the greatest influence on the Treaty reforms. Thirdly, it defines a new typology of intergovernmental conferences. The fourth is to evaluate the system reforms in the European Communities and in the EU in 1950-2024. Three theses were formulated for the book: (1) The IGC has become an institutional model of multilateral diplomacy of the EU; (2) The treaty law reforms have been the scene of numerous controversies between supporters of supranational and intergovernmental solutions; (3) The IGCs, as a forum for the participating States to play their national interests, were very often doomed to make difficult compromises.