This chapter explores the ways in which environmental concerns feature in International Rela-tions (IR) theory. It provides a brief introduction to the ecological crisis and the emergence of green theorizing in the social sciences and humanities in general, and then tracks the status and impact of environmental issues and green thinking in IR theory. It shows how mainstream ra-tionalist IR theories, such as neorealism and neoliberalism, have constructed environmental problems merely as a ‘new issue area’ that can be approached through pre-existing theoretical frameworks. These approaches are contrasted with more critical green IR theories, which chal-lenge the state-centric framework, rationalist analysis, and ecological blindness of orthodox IR theories and offer a range of new environmental interpretations of international justice, account-ability, development, order, and security. In the case study, climate change is explored to high-light the diversity of theoretical approaches, including the distinctiveness of different green ap-proaches, in understanding how the international is relevant to global environmental crisis.