While food, energy and health security have been used by international organizations (FAO, IEA, WHO) since the 1970s, the water security concept has been discussed by the water community only since 2000. With the reconceptualization of security since the end of the Cold War, water security has been considered in the context of national, international and human security. This chapter analyses the origin and evolution of the water security concept in the framework of different social science theories on security, including the securitization discourse. Water security is compared with other sectoral security concepts including the water, energy and food security nexus. Water security is also being examined as a human security and sustainability issue with different operational linkages. However, the securitization of water remains ambiguous if no extraordinary measures are taken by policymakers. Whether water security may serve as a guiding principle for water governance and management remains uncertain.