Abstract. 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 NO₂. 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. ClONO₂ has vibrational–rotational bands in the infrared, notably at 779, 809, 1293, and 1735 cm⁻¹, which are used for remote sensing of ClONO₂ in the atmosphere. Mid-infrared emission and absorption spectroscopy have long been the only concepts for atmospheric ClONO₂ measurements. More recently, fluorescence and mass spectroscopic in situ techniques have been developed. Global ClONO₂ 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 ClONO₂ concentrations are an indicator of chlorine activation and de-activation.