This book is about the regime of international watercourses; that is, the principles, norms, rules, and decision-making procedures regarding the access, use, sharing, and management of international watercourses. It focuses primarily on transboundary surface water, keeping in mind the inter-relationship of such water with other components of the hydrological cycle - namely, the watershed, the groundwater, atmospheric water, and the coastal and the marine waters - and the need for an ecosystemic approach in managing such waters to attain sustainable development.