This chapter reviews the fundamentals of low-temperature arsenic geochemistry that control arsenic concentrations in groundwater including arsenic abundance, aqueous and gas-phase speciation, arsenic mineralogy, and arsenic sorption behavior. It discusses two important arsenic problems: arsenic in mining, which probably represents the most significant dispersal mechanism of arsenic into the environment, and arsenic in Bangladesh, the most significant human impact of arsenic in the world. Researchers agree that the high concentrations of arsenic in the groundwater of Bangladesh is not caused by pollution but rather has been mobilized in situ from the sediments. Relative to the extent of research on biogeochemical processes of arsenic mobilization, little work has been conducted in Bangladesh to understand groundwater flow and how it transports arsenic and the solutes that control arsenic mobility. The chapter presents some hydrologic data that characterizes the annual pattern of groundwater circulation at our field site and indicates several important features of groundwater flow for arsenic mobilization and transport.