The cycle of nitrogen oxides in the troposphere is discussed from both global and regional perspectives. Global sources for NOx are estimated to be of magnitude 50(¿25)¿1012gmN yr-1. Nitrogen oxides are derived from combustion of fossil fuels (~40%) and biomass burning (~25%) with the balance from lightning and microbial activity in soils. Estimates for the rate of removal of NOx based on recent atmospheric and precipitation chemistry data are consistent with global source strengths derived here. Industrial and agricultural activities provide approximately two thirds of the global source for NOx. In North America, sources from combustion of fossil fuels exceed natural sources by a factor of 3--13. Wet deposition removes about one third of the combustion source of NOx over North America, while dry deposition removes a similar amount. The balance is exported from the continent. Deposition of nitrate in precipation over eastern Canada and the western Atlantic is clearly influenced by sources of NOx in the eastern United States. Major uncertainties in the cycle of NOx are attributed to lack of field measurements and inadequate understanding of heterogeneous processes involving NOx.