The regulation of atmospheric pollution is one of the earliest and best-developed areas of international cooperation and regulation in the fi eld of the environment. Atmospheric pollutants, by their very nature, easily cross national boundaries and may cause harmful environmental effects and, as a result, confl icts between states, creating the need for international agreements on how emissions are handled. In addition, some emissions are only problematic upon reaching harmful concentrations in the atmosphere, as in the cases of ozone depletion and climate change, introducing the further complication of allocating how and where reductions should be made and who should pay for them. Tackling these environmental and political crises is a critical challenge for the international community. The nature and evolution of global efforts to address atmospheric pollution are the focus of this chapter. As in other sub-fi elds of international environmental law, most rules regulating atmospheric pollution are found in treaties. Obviously, international customary rules apply, mutatis mutandis, to atmospheric pollution as well. An increasingly important role is also played by soft law.