Water is essential to all life on Earth. However, the availability of potable water is decreasing every day as a result of a growing population as well as increasing polution of the national and internationl watercourses. Scientists estimate that in the year 2025 one third of the world’s population will suffer under water stress or chronic water scarcity. For this reason, international disputes between states sharing an international watercourse are even more likely to occur in the near future. This book examines the concepts of Territorial Sovereignty, Sustainable Development, No-Harm and Equitable Utilization and discusses whether they qualify as principles of Customary International Law, and in what way they influence the area of International Water Resources Law. International regulation, case law and scholarly theory are studied as well as the role of regional basin regimes. Furthermore, the applicability of customary law principles on three fictitious case studies involving international conflicts relating to water resources is examined.