Abstract This thesis explores the contending role of multilateralism in today's inter-state relations. While the topic is a subject of major discussion between Neorealists and Liberals in the discipline of international relations and in real world politics, the present study hopes to contribute for a better understanding of multilateralism The study does so by investigating the International Thermonuclear Experimental Reactor (ITER) Project as a case study of multilateralism. ITER is a scientific programme developed by a group of states, which aims the harnessing of fusion energy as the path for creating an abundant source of energy. The research focuses on events in three different time frames: i) upon the establishment of ITER; (ii) the withdrawals and entries of new partners in the project; and (iii) the choice of the location to build ITER. Departing from John Ruggie's concept of multilateralism and theoretical understanding of international relations, the objectives of the research are to identify the multilateral elements of the cooperation and explain why partners to the project chose a multilateral framework. It also intends to find out why cooperation continued, or was resumed when some of the partners withdrew from ITER, and later, when cooperation broke down during the choice of the ITER site location The work is based on a detailed analysis of primary and secondary sources of information which include all ITER agreements. The investigation confirms that inter-state multilateralism is a very demanding framework, but it can also be highly resistant against disruptive conflicts.It finds that an inter-state authority of a multilateralized diplomacy embedded in international relations, particularly in the nuclear field, induced partners to choose a multilateral framework, though not in its pure form. Moreover, the thesis argues that the multilateral elements of ITER and the flexibility of the framework were crucial for collective cooperation to endure partners' withdrawals and the site dispute. Cooperation continued because partners share the common expectation of benefiting from ITER indivisible and ultimate good - the knowledge which is believed to be a small step for creating a cleaner and almost inexhaustible source of energy. If multilateralism in ITER proves to succeed and gives a positive contribution to solving the issues of pollution and energy, it would strengthen Liberal theories argumentation and represent a serious setback to Neorealism fundamentals.