Fresh water is one of man's most vital needs. The distribution of water within river basins has a direct bearing on the organization of water resources development to meet this ever-expanding need. River basins, despite their very great diversity in other respects, have one physical characteristic in common: each is a more or less self-contained unit within whose bounds all the surface and part or all of the ground waters form an interconnected, interdependent system. This inter- dependence has such far-reaching implications - for pollution and flood control, apportionment of supply, relations between upstream and downstream riparians, to mention only a few examples - that the river basin has become almost universally accepted (within the past 20 or 30 years at least) as the unit of optimal water resources de- velopment. Professor Teclaff's work (which was originally submitted to the New York University School of Law as a doctoral dissertation) is the first fully developed response to the important resolution passed by the International Law Association at its New York meeting in I958 recognizing the legal nature of the international river basin. His study quite properly, therefore, poses the question whether the adoption of the river basin unit is a temporary phenomenon, reflecting the current stage of technology and of administrative, economic, and legal thought on water resources development, or whether the de- terminative influence of the river basin's physical unity which has always operated in the past will continue to operate in the future.