The law applicable to transboundary waters is a corpus juris that dates back to the 19th century. It originally focused on regulating the uses of transboundary watercourses for navigation and commercial transport. It was crafted primarily on the European and North American continents, and it has gradually become universally applicable, thereby taking a new shape. The regulation of transboundary waters was rooted in a strict dynamic of coexistence between sovereign entities: each acted as it saw fit with respect to “its” portion of the watercourse, which was treated at the same time as the image of the territory to which it is attached. The need for regulation only arose when uses affected the riparian states’ exercise of their “sovereign rights.” Since the 1990s, the law has tried to break away from this “classical” logic to make room for more community-based and even “ecosystem” notions based on aspects of joint management, and sometimes even pool of shared resources. A number of treaties have been negotiated and adopted by states bordering transboundary watercourses in Europe, Asia, Africa, and the Americas. They reflect, and sometimes even develop, some of the principles and rules enacted in broader forums, such as the United Nations (UN) or its Economic Commission for Europe, or the European Union. These efforts show the steps taken in the field of transboundary waters management, but they also reveal some of its limits, as they do not always comprehend all facets of water management and protection.