Arsenic is a toxic element for animals and the majority of plants, in spite of evidence that it is also an essential element. The long-term intake of small doses of arsenic has a carcinogenic effect. There are well-identified regions where arsenic ground water concen- trations can reach values higher than 2 mg/L. Water purification and waste treatment techniques based on (1) precipitation of calcium, magnesium, and iron(III) arsenates, and/or (2) adsorption or coprecipitation of arsenic oxyanions are unlikely to produce aqueous solutions with arsenic concentrations below the guideline values proposed for arsenic dissolved in potable water and treated sewage effluents. As(III) species are more toxic than As(V) species. Arsenate species are predominant at moderate and high redox potentials, while arsenite species occur under more reducing conditions. Metal arsenites are much more soluble than the corresponding metal arsenates, and arsenites are adsorbed less by solid phases. Remediation techniques must consider the available information on solubility and adsorptive properties of As(III) and As(V). The less-soluble lead and barium arsenates are not suitable for arsenic decontamination. New remediation methods must consider solubility data for arsenic-containing materials and minerals.