Water, the liquid of life, like air and soil, has no substitute. Life, as we know it, would not exist without water. The geosphere, atmosphere, and biosphere are all tied to and through water. Water interacts with solar energy to determine climate and transforms and transports the physical and chemical substances necessary for all life on earth. Although 70% of the planet's surface is covered by water, most of it is salty; fresh water is limited and is not always in the right place, at the right time or of the right quality. Water, on Earth, exists in different states: water vapor and clouds in the sky, sea water in the oceans, icebergs in the polar oceans, glaciers in the mountains, freshwater lakes, rivers, and fresh and salt water in aquifers. There are significant continental and national differences in freshwater availability. Over the past 25 years, these differences and the importance of water have grown to occupy national and international agendas. Many international organizations, such as the United Nations, the World Bank, the World Health Organization, the World Meteorological Association and the Stockholm International Water Institute, hold conferences related to water issues. Awareness among scientists, political leaders and citizens of the connections between climate change, the hydrological cycle, food production, environmental services, infrastructure needs and sustainable management of water resources increases every year. The message highlighted by all these efforts is that water is an increasingly scarce resource and that it is important to recognize and accept that its availability is limited. Competition between agriculture, industry and cities for limited water resources is already limited... to the middle of the paper...). While floods and droughts have always plagued humanity and agriculture, they serve to raise awareness of water issues and the need to use and manage water resources wisely. As a prelude to sustainable water management for agriculture and food systems, this chapter briefly outlines water supply and use factors and challenges for the future. It begins by providing a brief overview of the physical and chemical uniqueness of water before discussing the location, availability, and uses of water. A section on "virtual water" provides information on the amount of water incorporated into foods, fibers and products. The impacts of drought and climate change on water availability are outlined. Before the conclusion, issues related to sharing and managing transboundary surface water and groundwater resources are highlighted. These problems will become increasingly complex with climate change.