Drawing upon the experience of a century, nations have constructed a customary international law for transboundary fresh water resources built around the principle of equitable utilisation. The earliest complete formulation of this body of law was as the Helsinki Rules on the Uses of International Rivers of the International Law Association of 1966. Like all customary law, this body of international Law retains flexibility by being vague while allowing only for relatively primitive enforcement mechanisms. In an effort to improve things, the United Nations, working initially through the International Law Commission, drafted a convention to codify the customary law. Even before that treaty enters into force, it has become the most cogent summary of the relevant customary international law. Despite certain advances over the customary law in the terms of the treaty, it ultimately fails adequately to integrate the environmental or ecological concerns now emerging in international law into the older body of international water law. This need suggests that a revised Helsinki Rules could serve to complete the unfinished task of adapting international water law to the needs of the twenty-first century.