The world's water is one of our most precious resources. Lack of water is one of the principal causes of delayed development. Polluted water is one of the biggest killers we know, responsible for up to 27000 deaths a day in the world's poorest countries.Of the limited volume of freshwater that is available to us, some 97 percent (excluding permanently frozen water) is stored underground. More than 1500 million people rely on this groundwater for their drinking water. Farmers all over the world use it to irrigate their crops. In arid areas, where rainfall is low or virtually non-existent, groundwater may be the only source of water for the human population.As populations grow and their need for water increases, the pressures on our groundwater resources also increases. In many areas of the world, groundwater is now being over-abstracted, in some places massively so. The result is falling water levels and declining well yields, more expensive supplies, land subsidence, the intrusion of salt water into freshwater supplies and ecological damage such as the drying out of wetlands.Groundwater is also being polluted. Cities with poor sanitation systems are allowing foul water to seep into underground aquifers, whence it eventually contaminates the boreholes and wells that supply drinking water. Industries accidentally spill or release their effluents into the ground or into surface water courses whence pollution is carried deep underground. Chemical fertilizers and pesticides are leached through the soil and down into the aquifers as a result of the intensification of agriculture.The end result is a serious deterioration of groundwater quality. It is always extremely difficult and very costly to clean up a polluted aquifer. Often it is simply impossible. Because water flows so slowly underground, it takes many years or decades for pollution to show up. Many of the aquifers from which we are currently abstracting pure water may already be contaminated.It is therefore urgent that we protect our groundwater supplies with diligence, and that everyone understands the importance of doing so. This publication is intended to explain to readers how important groundwater is, the extent of the threat to it, and the actions that can be taken to protect it.