Arsenic poisoning has three types of poisoning. First, acute arsenic poisoning is usually caused by oral intake of large amount of arsenic compound with purpose of homicide or suicide. Second, chronic arsenic poisoning is caused by inhalation of arsenic in the occupational setting or by long-term oral intake of arsenic-contaminated well water. Third, arsine poisoning occurs acutely when impurities of arsenic in non-ferrous metal react with acid. Clinical manifestation of acute arsenic poisoning is mainly gastrointestinal symptoms and cardiovascular collapse. Those of chronic poisoning are skin disorder and cancer. Arsine poisoning shows massive intravascular hemolysis and hemoglobinuria with acute renal failure. Exposure evaluation is done by analysis of arsenic in urine, blood, hair and nail. Species analysis of arsenic is very important to evaluate inorganic arsenic acid and monomethyl arsenic acid (MMA) separated from dimethyl arsenic acid (DMA) and trimethyl arsenic acid (TMA) which originate from sea weed and sea food. Treatment with dimercaprol (BAL) is effective in acute arsenic poisoning only.