Rapid and extensive industrialisation in many states in the nineteenth century generated serious atmospheric, terrestrial, riverine, and marine pollution highly damaging to human health and the natural environment. In the early twentieth century these problems took on transboundary dimensions as pollutants crossed borders and damaged the environment of other states and global commons areas, and this proved an important catalyst for developments in international environmental law. It was atmospheric pollution originating in Canada, and damaging to agricultural interests in the United States, that led to the seminal Trail Smelter case in which an arbitral tribunal concluded that no state has the right to use, or permit the use of, its territory in such a way as to cause serious injury by pollution in the territory of another state. There is no more elemental stipulation of customary international law relating to environmental questions.

This chapter examines the critical contribution that arbitral awards and judicial decisions have made to the evolution of legal principles relating to transboundary environmental damage. It is seen that this field of law did not spring from environmental concerns per se, but rather from a more general and protean notion that states should be protected from unreasonable interference. Despite the substantial evolution of this area of law since the Trail Smelter case, especially through the work of the ILC, it remains tied to some of its problematic conceptual origins as a body of principle concerned primarily with the limits of state sovereignty rather than positive duties of environmental protection.