Editors' Summary: Rivers, lakes, and aquifers cross national borders around the world creating international interdependencies related to one of the world's most precious resources. More than one-half of the world's population derives their water from international sources, located beyond the jurisdiction and control of the country where they live. What are the rules of international law that govern these shared waters, and how can national water policy objectives be pursued in light of such interdependency, especially in a world of sovereign states? In this Article, Dr. Patricia Wouters identifies the legal regimes that apply to international watercourses and uses Europe as a regional case study to compare these different regimes. She uses a five-point analytical framework to identify, examine, and compare the rules of international law that govern these shared waters. The Article concludes by highlighting the legal innovations at the heart of the dual-track governance regime that has evolved to regulate Europe's transboundary waters and embeds this study in the global context.