The long irrigation tradition in Spain excelled during the time of the Arabs, who introduced new techniques and the concept of collective management in the governance of the irrigation water. The water users' associations in Spain are firmly based on the effective structures that were developed more than seven centuries ago. Therefore, the new irrigation policies are directed towards the modernization of infrastructures rather than towards institutional reforms. On the other hand, researchers and engineers are emphasizing the need for developing assessment tools as part of the modernization plans. The irrigation networks are facing the dilemma of introducing new and costly changes in the physical infrastructure against introducing management tools for assuring irrigation sustainability, with changes in the physical infrastructure as a secondary option. In this paper are summarized the structure and organization of the water users' associations in Spain and, then, are presented two cases where an analysis of water use and of irrigation performance was carried out using newly developed tools.