 More than 200 million people worldwide are chronically exposed to arsenic. Arsenic is able to gain access to the developing brain and cause neurotoxic effects. Sex, concomitant exposures, and timing of exposure appeared to modify the developmental neurotoxicity of arsenic. The published literature indicates that arsenic is a human developmental neurotoxicant. This article was authored by Molly Tolins, Mothore's Richard Watt, and Philip Landrigan.