Although the International Law Association (ILA) was established in 1873, it only turned its attention to the internationally shared water resources in 1954, when its half-century study of the applicable principles and rules of international law thereon began. The first ILA committee assigned to this task was the Rivers Committee, which, after a decade of intensive study and through several resolutions and statements, arrived unanimously at a set of articles reflecting customary international law, known as the Helsinki Rules on the Uses of the Waters of International Rivers.The Helsinki Rules, approved at the ILA 1966 Helsinki Conference, were soon widely accepted across the Globe as a non-binding authoritative source of international water law. This monograph traces the work of the ILA leading to the Helsinki Rules, analyses the Rules, and identifies their influence on and contribution to the evolution of international water law.