Arsenic concentrations in shallow, reducing groundwaters in Bengal, Southeast Asia, and elsewhere constitute a major hazard to the health of people using these waters for drinking, cooking, or irrigation. A comparison of occurrences in the Ganges-Brahmaputra, Mekong, and Red River basins shows that common geological characteristics include (1) river drainage from the rapidly weathering Himalayas, (2) rapidly buried organic-bearing and relatively young (ca. Holocene) sediments, and (3) very low, basin-wide hydraulic gradients. Anaerobic microbial respiration, utilizing either sedimentary or surface-derived organic carbon, is one important process contributing to the mobilization of arsenic from host minerals, notably hydrous iron oxides. In spite of the paucity of data from before the extensive development of groundwater pumping in these areas, there is sufficient evidence to make a prima facie case that human activity might exacerbate arsenic release into these groundwaters. The difficulties in implementing comprehensive groundwater remediation suggest serious attention should be given to developing treatment technologies for alternative surface-water supplies.