Abstract. The article addresses the use of complexity theory in the analysis of international relations. Complexity theory points to the inherent unpredictability of world relations theories. The current international system has reached such complexity level of politics that it cannot be analyzed on the basis of linear rationality used in the international standard, which leads to non-deterministic causality. The article discusses the evolution and basic tenets of the complexity theory, approaches to the world politics analysis established within its framework. The complexity research methodology focuses on actors and their values, interests and beliefs, as well as on the nature of interactions between them. In this regard, complexity theory is closely related to the modern constructivist theory of international relations. Today, the number of international actors is increasing, which increases the complexity of the world system. Therefore, analytical methodology should take into account the role of non-state actors as well as the high complexity of contemporary world politics, which is multi-layered and dynamic. In this respect, the complexity theory is associated with contemporary neoliberalism. Agent-oriented computer-based modeling is the main and a very promising scientific methodology applied to the study of complex adaptive systems, including world politics. In the complexity theory, this modeling implies the simulation of agent behavior (in this regard, agents are international relations actors), based on the simulation of the patterns, according to which agents process the information using adaptive mechanisms or behavior limiting norms and rules. In general, in terms of the complexity theory, foreign policy issues are always multidimensional, decisions have unintended consequences and are never simple. However, complex systems can be controlled, and even their structure can be altered. Still, there are no unambiguous tools of influencing the situation, and all recommendations should be taken with caution. The authors conclude that the complexity theory offers new explanations, research directions and practical perspectives for international relations research. Agent-oriented computer simulation also allows the incorporation into the analysis of a significant part of the knowledge accumulated in the international relations traditional theory framework.