This paper, written for a forthcoming volume on climate change, provides a positive analysis of the roles and impact of non-state actors (NGOs) in the global climate change regime. It first identifies some of the major actors and distinguishes between two main types of nonstate actors: NGOs and epistemic or expert/scientific communities. Special attention is given to a uniquely important scientific, though not purely nongovernmental, body: the Intergovernmental Panel on Climate Change (IPCC). We then describe and analyze the activities of nonstate actors, examine their influence on current international responses to global climate change, and assess the significance of their current prominence for theories of international cooperation.