Among the most serious problems in water resources is the degradation of groundwater. The objectives of IHP-V Project 3.1, ‘Groundwater contamination inventory’ were to document the extent, spatial distribution and types of water contamination— including point-source and non-point-source problems, as well as natural contamination due to saline water intrusion at regional scales — while developing a standardized methodological guideline. Data on the extent of contamination were gathered by the IHP National Committees and the UNESCO Regional Offices of Science and Technology. The methodological guideline on groundwater contamination inventory is presented in this publication.