The release of arsenic from geothermal systems into surface and ground waters compromises the use of these waters as drinking water resources. In surface waters, As contamination can also adversely affect aquatic ecosystems, accumulating in sediments and plants. This review examines the source of arsenic in geothermal areas, its transport and speciation in geothermal fluids and receiving waters, as well as the deposition and removal mechanisms occurring in both natural environments and waste or water treatment systems. The effect of microorganisms on As mobility, and the opportunities that exist for further research in this field, are discussed. The review focuses on two geothermally active regions which have been intensively studied: Yellowstone National Park in the USA, and the Taupo Volcanic Zone in New Zealand, and their associated catchments.