Arsenic (As) contamination, especially in groundwater, has been receiving increasing attention in recent years due to its adverse effects on human health and the environment. Its biogeochemical cycle involves several physico-chemical processes as well as biological mechanisms, in which microorganisms play a key role. The inorganic compounds arsenite [As(iii)] and arsenate [As(v)] are the most toxic and abundant species of arsenic in water. Depending on the environmental physico-chemical condition these compounds have a relatively high solubility and are readily transported through aqueous routes into the environment, transferred from soils to crops and accumulated in various food crops and aquatic plants, threating human health. According to recent studies rice may be the primary source of inorganic arsenic for human exposure. In this chapter sources, pathways and levels of arsenic are presented to address the problem of its contamination of the environment. Furthermore, the state-of-the-art treatment of arsenic-contaminated waters, including a variety of treatment technologies based on oxidation, coprecipitation, adsorption, ion exchange and membrane and bio-processes are reported. The efficiency and applicability/appropriateness of the technologies have been evaluated, with regard to influent arsenic concentration, differences in source water composition, production of toxic sludge, economical aspects and social acceptance.