Nitrate is one of the major impurities in polar snow, and is relatively easy to analyse. Large amounts of data therefore exist, including some from cores extending into the last glaciation. However, the data are not easy to interpret, and we do not yet have an adequate knowledge of even the present-day sources of nitrate to polar snow, nor of the deposition processes that control the concentrations seen. It is clear that anthropogenic pollution has increased the concentrations in Greenland snow by a factor of two in recent decades, and that no similar increase is seen in Antarctica. In pre-industrial Greenland ice, a clear seasonality allows annual layer counting. The sources in pre-industrial ice are probably lightning and/or the stratosphere, while soil exhalation may be an additional major component in Greenland. Whereas nitrate in Holocene ice is present as nitric acid, in ice from the last glaciation it is present as neutral salt, associated with terrestrial cations