In the majority of southern Mediterranean countries, the structural imbalance between the constantly increasing demand for water to meet needs and the natural available water resources will be apparent around the year 2000. In this situation, economics in the use of water in the sector of agriculture, industry and build-up areas is vital in the arid and semi-arid countries over the next few decades. Any action aimed at economy in the use of water will have an important impact in preventing the destruction of the basic structures of development. In such countries, the suistainability of the development system can only be the adoption of an long-term strategy which brings together the physical, economic and social factors. This strategy must be based on a dynamic evaluation of the underground and surface water resources by analysing their regime and behaviour. This evaluation must take account of the occurence of exceptional droughts. National water policy should be prepared to guide the harnessing and use of water by comparing short and long-term resources adopting several hypotheses and cutting forward several alternatives. The recycling used water could lead to a great deal of progress, but is still limited by the lack of research in this field. This approach appears to have a great future and will enable the impact of scarcity to be minimized in times to come.