A healthy water sector has been identified as a crucial contributing factor in reaching and maintaining a healthy society, from an environmental, human health and economic perspective. However, freshwater resources are under increasing pressure and climate change is expected to have a substantial impact on the hydrological cycle and freshwater resources. Considering the fact that the territories of 145 States worldwide, and 60 per cent of EU territory, run through transboundary lakes and river basins, their good governance is essential in tackling the challenges ahead. To this end, solid inter-regional and international upstream-downstream cooperation mechanisms are necessary. The Law of the River takes a comparative perspective by analysing the relevant legal and policy frameworks in the European Union and the United States, with the goal of providing Critical Success Factors to transboundary river basin management. Each Critical Success Factor has resulted in a specific set of recommendations, both geared toward the level of the EU and the International River Basin Districts. The main focus of this book is on water quantity management, specifically floods, scarcity and droughts. The relevant EU requirements for water quantity management in transboundary waters (International River Basin Districts) and the way these have been implemented in a specific river basin in the EU, the Scheldt, are scrutinized. Moreover, a case study of the river basin mechanism governing the Delaware River in the United States has been conducted with the aim of identifying lessons learned for the EU. The key theme running through the book is based on the social-ecological resilience theory and the water security paradigm. This book proposes an Enhanced Cooperation Model, whereby the notion of "River Basin District Security" is set forth as a comprehensive assessment tool to be used in transboundary river basins.