This book focuses on River Basin Organizations as the key institutions for managing internationally shared water resources. This includes a comparative analysis of all River Basin Organizations worldwide and three in-depth case studies from three different continents. The detailed case studies are the Senegal (West Africa), Mekong (South-east Asia) and Danube (Europe) rivers. The book contributes to the academic debate on how shared natural and environmental resources can be managed in a sustainable way and which institutional and legal mechanisms actually matter for doing so. It adopts the neo-institutionalist approach, according to which international environmental institutions do make a difference. The analysis not only confirms this argument for the specific case of shared water resources, but also refines existing hypotheses on the influence of different independent variables, namely the nature of the collective action problem, the constellation of actors and the institutional design of an international environmental institution. The work also contributes to the policy debate on how to better govern internationally shared natural resources and the environment. It provides policy makers with advice on which exogenous conditions to be aware of when managing water resources they share with co-riparians and which institutional design features and governance mechanisms to set up in order to increase effectiveness in management.