Nitric oxide, or nitrogen oxide, also known as nitrogen monoxide, is a molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry. Nitric oxide is a by-product of combustion of substances in the air, as in automobile engines, fossil fuel power plants, and is produced naturally during the electrical discharges of lightning in thunderstorms. In mammals including humans, NO is an important cellular signaling molecule involved in many physiological and pathological processes. It is a powerful vasodilator with a short half-life of a few seconds in the blood. Long-known pharmaceuticals such as nitroglycerine and amyl nitrite were found to be precursors to nitric oxide more than a century after their first use in medicine. Low levels of nitric oxide production are important in protecting organs such as the liver from ischemic damage. Nitric oxide should not be confused with nitrous oxide (N2O), an anaesthetic, or with nitrogen dioxide (NO2), a brown toxic gas and a major air pollutant. However, nitric oxide is rapidly oxidised in air to nitrogen dioxide. Humphry Davy discovered this to his discomfort, when he inhaled the gas early in his career. Despite being a simple molecule, NO is an important biological regulator and is therefore a fundamental component in the fields of neuroscience, physiology, and immunology. It was proclaimed “Molecule of the Year” in 1992. Research into its function led to the 1998 Nobel Prize for discovering the role of nitric oxide as a cardiovascular signalling molecule.