Given the vital role of water in civilization, multinational water bodies have long been a subject of international law, developing over the centuries a relatively rich body of international customary law. The UN Convention is intended to be a “framework” treaty (5th preamble), establishing general legal principles which could lead to further refinement in protocols and multilateral agreements on specific watercourses. Europe, with many more centuries of watercourse-related development, has an immense array of international freshwater treaties. The Helsinki Convention, while compatible with the 1997 UN Convention on International Watercourses, is both narrower and broader. International environmental law and policies are slowly but surely being integrated into international water law.Customary international law and the modern treaties now recognize a number of important principles with regard to internationally shared water resources.  Keywords:1997 UN Convention, customary international law, groundwater, international freshwater, international freshwater resource, international watercourse