This proceedings, ""Groundwater: An Endangered Resource"", contains papers presented at the 27th Congress of the International Association of Hydraulic Research (IAHR) entitled ""Water for a Changing Global Community."" In many places in the world, groundwater is threatened either by excessive use or by contamination resulting from industrial activities, agricultural practices and waste disposal methods. Over the last two decades many researchers and practitioners have concentrated their efforts on addressing these issues. This proceedings brings together some of the results of these efforts. The papers cover six general topics: heterogeneous aquifers; groundwater remediation and risk management; stochastic approaches to subsurface hydrology; drainage water relationship to water quality; groundwater modeling; and water and contaminant movement in porous and fractured media.