The significance of water availability for sustainable development is discussed. A review of water-related areas of concern in Agenda 21, the blueprint for sustainability, is offered, with particular reference to its Chapter 18 devoted to freshwater resources. The problems of water resources of vulnerable areas are tackled for the examples of arid and semiarid lands, mountains and small islands. The importance of water resources assessment and its building block, hydrological observations, for sustainable development is demonstrated and the adverse tendencies of declining hydrological observation networks are reported. Hydrological network density is seen as one of the indicators of sustainable development. Hydrological and water resources systems are perceived only as a component of the complex global system. Therefore a holistic perspective is advocated, looking at the existing interrelationships and interfaces with other sub-systems, including social, economic, institutional, etc.