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. Aquifers, among them numerous transboundary ones, are coming under growing pressure from over-abstraction and pollution, which seriously threaten their sustainability. Up to now international law has paid much less attention to ground- 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. This publication brings together binding and non-binding international law instruments that, in varying degrees and from different angles, deal with groundwater. Its aim is to report developments in international law and to contribute to detecting law in-the-making in this important field.