The knowledge we have of available water, though inadequate in many ways, has proved useful so far. However, the knowledge about availability, variability, reliability and quality has to become more precise. In the face of population growth and the increasing demand for water, water quality deterioration, increasing environmental degradation and impeding climate change, more effort is required to assess water resources for rational planning and management to sustain development. Those efforts should be directed to overcome the present constraints regarding water resources assessment and in particular the institutional weakness, inadequate networks, incompatible technologies for field, laboratory and office work. Deficiency of staff and their capability and the lack of coordinated, relevant research have constrained efforts further. Challenges and opportunities for water resources development in the Mediterranean countries call for institutional reforms, improvements in the knowledge of hydrological processes and increase in funding, capacity building and international cooperation on national, sub-regional, regional and global levels.