Sulfur mustard is a chemical warfare agent which was widely used during World War I and more recently in conflicts in the Middle East. This highly toxic compound causes severe dermal, gastrointestinal, respiratory and ocular injuries. It acts as an alkylating agent that induces structural changes and, hence, destruction of nucleic acids and proteins, impairing the cell's normal homeostasis and eventually causing its death. Sulfur mustard reacts rapidly with ocular tissues, and after a latent period of a few hours the patient starts suffering from severe eye pain, photophobia, excessive lacrimation and blindness. The injury, which is restricted to the anterior segment of the eye, may cause long-lasting incapacity in large numbers of casualties. Approximately 0.5% of the severely wounded victims may develop late complications which require prolonged ophthalmologic observation and therapy. In light of the ever-present threat of mustard chemical warfare against military and civilians, physicians worldwide should be aware of its grave effects and know how to care for its victims.