This chapter turns to issues related to fresh water. Fresh water is a finite resource and the more we pollute it, the more issues we have with its use. A sustainable supply of fresh water is vital to life. Historically, international water law was not particularly concerned with environmental problems. This chapter talks of ‘international watercourse’ which is a convenient designation for rivers, lakes, or groundwater sources shared by two mor more states. The law of international watercourses has for most of its history been concerned with the allocation and use of a natural resource of international significance, not with its conservation or environmental protection. While it can be asserted with some confidence that states are no longer free to pollute or otherwise destroy the ecology of a shared watercourse to the detriment of their neighbours or of the marine environment, definitive conclusions concerning the law in this area are more difficult to draw.