This work evaluates the risks to human health and the environment posed by arsenic and arsenic compounds. Arsenic is widely distributed in the Earth's crust and is emitted into the atmosphere by coal-fired power generation plants and volcanic activity. Inorganic arsenic of geological origin is found in groundwater used as drinking-water in several parts of the world, for examples Bangladesh. In these areas, drinking-water is the main source of arsenic intake, but elsewhere food is the principal source. A selection on kinetics and metabolism notes that arsenic in hair and nails can be indicators of past arsenic exposure. Arsenic in hair may also be used to estimate relative length of time since an acute exposure. The second half of the report evaluates the large number of investigations of arsenic toxicity. The section concerning effects on human health notes that soluable inorganic arsenic is acutely toxic and ingestion of large doses leads to gastrointestinal symptoms, disturbances of cardiovascular and nervous system functions and eventually death. In survivors, bone marrow depression, haemolysis, hepatomegaly, melanosis, polyneuropathy and encephalopathy may be observed. Long-term exposure to arsenic in drinking-water is causally related to increased risks of cancer in the skin, lungs, bladder and kidney, as well as other skin changes such as hyperkeratosis abd pigmentation changes. Chronic arsenic exposure in Taiwan has been shown to cause blackfoot disease, a severe form of peripheral vascular disease which leads to gangrenous changes. In the ecological section, evidence is presented that arsenic causes effects including inhibition of growth, photosynthesis and reproduction and behavioural effects. Arsenic-contaminated environments are characterized by limited species abundance and diversity. The report concludes with a number of recommendations for future research.