Since 1981, three groups have reported spectroscopic detections and measurements of hydrogen cyanide in the atmosphere, HCN concentrations (volume mixing ratios) of (1.5-1.7)¿10-10 appear to characterize the stratosphere and the northern hemisphere's nonurban troposphere. In this paper we explore the atmosphere behavior of HCN by examining its chemical and photochemical properties. Its principal sinks are reactions with atmospheric OH and O(1D): precipitation appears to be a negligible sink. In the stratosphere, vacuum UV photons also attack HCN. Atmospheric model calculations show that HCN should be relatively well mixed in the troposphere and that its concentration decreases slowly with altitude in the stratosphere. Its atmospheric residence time appears to be about 2.5 years, although 1.5 years in a possible range. To maintain the observed burden of HCN, an annual source of about 2¿1011 g nitrogen as HCN is required; we speculate as to the identity of these sources. Oxidation of HCN by OH2 while the major sink for atmospheric HCN, is not simple or direct. Instead, oxidation proceeds from the HCN OH adduct formed in HCN+OH reactions. These pathways and their uncertainties are outlined here.