The contamination of groundwaters, abstracted for drinking and irrigation, by sediment-derived arsenic, threatens the health of tens of millions worldwide. Microbial processes are accepted as playing a key role in arsenic mobilisation from sediments into groundwaters, but the precise biogeochemical mechanisms remain a subject of debate. A combination of field investigations, coupled to laboratory experimentation with sediment samples using “microcosm” approaches, has provided a significant body of evidence supporting a key role for anaerobic metal-reducing bacteria in the reductive mobilization of arsenic in aquifers in West Bengal, Cambodia, Vietnam and Bangladesh. The application of high-throughput next generation sequencing, combined with metagenomic reconstructions and other “omics” techniques from the life sciences is shedding new insight into the processes at play, and identifying new mircoorganisms and coupled biogeochemical processes that control the solubility of arsenic in Asian aquifers.