Groundwater is of high social, economic, environmental and strategic importance. It represents about 97% of the freshwater resources available on earth, excluding the water locked in the polar ice caps. Aquifers, among them numerous transboundary ones, are coming under growing pressure from over-abstraction and pollution, which seriously threaten their sustainability. Until now, international law has paid much less attention to groundwater than to surface water. Slowly, however, a body of rules dealing with this vital resource is emerging that indicates a trend towards more comprehensive international regulation. It is against this backdrop that FAO and UNESCO have joined forces and embarked on this publication project. It brings together binding and non-binding international law instruments that, deal with groundwater in varying degrees and from different angles. Its aim is to report developments in international law and contribute to anticipating future law in this important field.