Arsenic is an element known throughout history as a classic poison. Currently, very small but highly significant concentrations of this element in drinking water supplies are causing massive health problems to many millions of people in some of the world’s poorest nations. More localised sources related to mining and processing are also a concern. A review of background information on arsenic chemistry, occurrence in the Earth, production and uses, as well as its toxic properties, leads in to the other articles in this issue of Elements.