Water resources in Mexico are threatened by scarcity, pollution and climate change. In two decades water consumption doubled, producing water stress in dry seasons and semi-arid and arid regions. Water stress rises due to physical and economic stress. In seven parts a multidisciplinary team analyzes hydrological processes in basins and their interaction with climate, soil and biota. Competing water use in agriculture, industry and domestic needs require savings, decontamination processes and desalination to satisfy the growing demand. Water quality affects health and ecosystems. This creates conflicts and cooperation that may be enhanced by public policy, institution building and social organization.