The lack of cooperation along river-sea corridors stands as a major obstacle to achieving sustainable development in these regions. In response, we have witnessed the emergence of various cooperation initiatives at the corridor level over the past 15 years. Nevertheless, implementing effective governance has proven complex, as illustrated in this chapter through the case of the Seine Valley. Despite numerous initiatives by various stakeholders since 2008, tangible results have been scarce, due to a number of institutional, geopolitical, andideological challenges.