Environmental and occupational exposure to arsenic is associated with increased risk of skin, urinary bladder, and respiratory tract cancers. The mechanisms responsible for arsenic carcinogenesis have not been established. Arsenic does not act through classic genotoxic and mutagenic mechanisms, as do other metals such as cadmium or chromium. Increasing evidence indicates that arsenic acts at the level of tumor promotion by modulating the signaling pathways responsible for cell growth.