This chapter initially delineates the essence of international energy cooperation from two perspectives: the agents and objectives of such cooperation. It elaborates on the nuances of energy cooperation, particularly emphasizing that unlike fossil fuel cooperation which primarily focuses on resource collaboration, renewable energy cooperation pays more attention to its manufacturing characteristics. This includes emphasizing cooperation across all aspects of supply and industry chains, adopting multifaceted and comprehensive approaches. A key element in regional energy cooperation is the establishment of systems such as trade liberalization. Subsequently, the chapter highlights the richness, diversity, multiplicity, and variety of international energy cooperation through examining four dimensions: content, agents, goals, and modes of cooperation. It discusses various patterns of cooperation including horizontal, vertical, integrated, and composite models among nations, institutions, and multinational corporations. Furthermore, the chapter systematically reviews theories of cooperation from multiple disciplines, including political science, economics, sociology, anthropology, biology, and law, and analyzes their implications for international energy cooperation. Finally, it provides a comprehensive overview of cutting-edge issues and methodologies in the research of international energy cooperation.