Groundwater is nearly ubiquitous in the subsurface portion of the ecosystem under the • ground surface of continents and islands. Groundwater exists in a complex subsurface environment, is mobile, and travels slowly, • but, in some cases, long distances across many boundaries. Managing groundwater on an aquifer basis in a watershed recognizes the groundwater fl ow • component of the hydrologic cycle and provides a holistic approach to addressing water issues. It represents the largest freshwater source of water supply.• Many organisms live in and rely on groundwater and the subsurface as their habitat.• The largest uses of groundwater are for water supply for human beings (in some cases, • their only water source) and other life forms (for some, their only habitat) and for irrigation (the largest use) to supply food-in many locations, of necessity, the only affordable source. Groundwater is a signifi cant resource input to the economies of communities and nations • around the world, supplying at least half the world’s population with water. Aquifers are under stress from depletion and contamination around the world, which will • most likely increase with climate change. Groundwater is vulnerable to contamination and is expensive to clean up.• Not as readily recognized is the use of groundwater and the subsurface as a waste • sink. Laws provide the basis for use of groundwater relative to who may access and use it, which • provides the basis for economic transactions affecting groundwater. Laws in some countries and jurisdictions treat groundwater as a separable water source, • but other countries recognize its role in the larger hydrologic cycle, such as in maintaining the basefl ow of streams. In some locations, groundwater is easily accessed but this is more diffi cult in other places-• this affects the cost to produce it, typically through wells. Aquifers around the world are being depleted-used up faster than they are recharged-• but have been instrumental through irrigation in ensuring food supply in many countries, especially some densely populated countries. Because of their many purposes and uses, groundwater and its subsurface environment • should be valued as some pecuniary amount signifi cantly greater than zero.