Vesicants (or blister agents) are cytotoxic alkylating compounds, which are chemical agents sometimes collectively known as mustard gas or simply as mustard. Other blister agents are nitrogen mustard; sulfur mustard; lewisite, a vesicant that contains arsenic; and phosgene oxime, a halogenated oxime that possesses different properties and toxicity from the other agents. This article discusses history, toxicity, clinical presentation, and common treatment for vesicants.