This review article compiles the characteristics of the gas chlorine nitrate and discusses its role in atmospheric chemistry. Chlorine nitrate is a reservoir of both stratospheric chlorine and nitrogen. It is formed by a termolecular reaction of ClO and NO 2 . Sink processes include gas-phase chemistry, photo-dissociation, and heterogeneous chemistry on aerosols. The latter sink is particularly important in the context of polar spring stratospheric chlorine activation. ClONO 2 has vibrational-rotational bands in the infrared, notably at 779 cm −1 , 809 cm −1 , 1293 cm −1 , and 1735 cm −1 , which are used for remote sensing of ClONO 2 in the atmosphere. Mid-infrared emission and absorption spectroscopy have long been the only concepts for atmospheric ClONO 2 measurements. More recently, fluorescence and mass spectroscopic in situ techniques have been developed. Global ClONO 2 distributions have a maximum at polar winter latitudes at about 20–30 km altitude, where mixing ratios can exceed 2 ppbv. The annual cycle is most pronounced in the polar stratosphere, where ClONO 2 concentrations are an indicator of chlorine activation and de-activation.