This chapter discusses the international liability of single major greenhouse gas (GHG) emitters. It demonstrates that the international climate regime is not equipped with a liability mechanism and that the current approaches to state liability in international law are incapable of addressing climate change-related damage. Obstacles to domestic climate change liability are also identified. The chapter then goes to show that both States and private enterprises – even those operating within or falling under an emissions trading system (ETS) – are bound by the international customary law obligation to prevent significant transboundary harm. It examines how the procedural and substantive duties related to this obligation can provide the legal basis for challenging single emissions sources.