This chapter discusses hydrocyanic acid, a poisonous solution of hydrogen cyanide (HCN) in water. HCN is a protoplasmic cytotoxic poison that acts as a corrosive on mucosa. Cyanide gas is absorbed from the respiratory tract. This is most rapid of all poisons in which death occurs from respiratory failure. The principle of treatment is to reverse the cyanide-cytochrome combination by converting hemoglobin to methemoglobin by giving nitrites. In this case, two 20 ml ampoules of 1. 5% dicobalt tetracemate is given I. V. followed by 20 ml of 50% glucose. In case of KCN/NaCN poisoning, it is advisable to perform gastric lavage with 5-10% solution of sodium thiosulphate, followed by potassium carbonate to form prussian blue. In case of suspected cyanide poisoning, cranial cavity should be opened first, as the odor of bitter almonds is well marked in the brain tissue. In some countries, hydrocyanic gas is used for legal execution. Prisoners are advised to take deep breaths after the gas is released, as this will considerably shorten their suffering.