Water plays a key role in the development and functioning of society by serving as a basic resource for activities such as irrigation, livestock production, fisheries, aquaculture, and hydroelectric power. Adequate water use in house-holds, businesses and manufacturing is a prerequisite of economic growth. Since many of the world's diseases are waterborne, we need clean water and sanitation for reducing the incidence of these diseases. And, most significantly, water provides habitat and sustenance for a rich diversity of plant and animal species that make up aquatic and riparian ecosystems, providing the basis for many of the goods and services received by society. Society is forcing unprecedented changes on global water resources through worldwide abstraction and pollution of water. Society also has a pervasive indirect impact because anthropogenic greenhouse gas emissions are causing long-term global changes in weather extremes and climate. Changes in the global water system are difficult to understand with simple cause-effect relationships because of the intense and complex linkages and feedbacks between different parts of the system. These changes and linkages also sometimes lead to abrupt changes in water systems such as the eutrophication of coastal aquatic systems, loss of biodiversity, the exceedance of safe water supply in urban areas, or intense competition between different water sectors for remaining water resources.