The Nile River basin is the home of more than 400 million people across eleven different states. It is the essential source of water for domestic, industrial and agricultural use in downstream countries, particularly Egypt, Sudan and Ethiopia. Yet for more than a century, there have been conflicting claims to the use and resources of the Nile watercourse and a series of legal responses to these claims. This book examines the multifaceted regulation of the Nile, starting with the Anglo-Ethiopian Treaty of 1902. It scrutinizes the non-treaty based claims of rights of the basin states to the Nile waters, examining the relevant rules of customary international law that set out rights and riparian obligations in the utilization of shared watercourses. This includes a comprehensive outline and assessment of contemporary riparian positions in Egypt, Sudan and Ethiopia, with a view to understanding the origin, essence and legal status of the key concept of 'natural-historical rights'. After reviewing developments since the 1990s, the legal and conceptual impediments that held back progress in negotiations on the Nile Basin Cooperative Framework are presented. The author shows how the essence of riparian rights of the Nile basin states hinges on how profoundly the legal standard so embedded in the 'equitable and reasonable utilization' principle has been entrenched in contemporary legal relations to define and constrain the discretion of the basin states.