This paper proposes to switch from a water stock to a water flow paradigm and therefore elaborate a new definition of water security. When seen and analyzed as a flow, it becomes possible to treat water as a substance with which a great variety of actors interact over a great variety of scalar levels. The paper also proposes a methodology to explore water security according to this new definition. The spatial, institutional, and sectoral trajectories that water follows both above and below ground are emphasized from the point where it emerges to the point where it disappears. After a brief review of existing governance systems in Israel and the West Bank, water security is analyzed in three Palestinian case studies: wells and springs in Al Far’a Valley; wastewater reuse; and irrigation in the Jordan River basin. Whereas past negotiations between Israeli and Palestinian authorities have focused on securing a stock of water, this paper concludes that reformulation of water security in terms of flows of water will open broader possibilities for forms of water governance that will promote sharing of transboundary water and ensure water security for all parties.