This chapter examines how environmental concerns have influenced International Relations theory. It first provides a brief overview of the ecological crisis and the emergence of green theorizing in the social sciences and humanities in general, along with the status and impact of environmental issues and green thinking in IR theory. It then investigates green theory’s transnational turn and how it has become more global, while critical IR theory has become increasingly green. It also considers the different ways in which environmental issues have influenced the evolution of traditional IR theory. It concludes with a case study of climate change to illustrate the diversity of theoretical approaches, including the distinctiveness of green theories.