Photochemical oxidants are found in 'photochemical smog' which is a complex mixture of primary and secondary air pollutants. The photochemical oxidants are secondary air pollutants formed by the action of sunlight on nitrogen oxides and reactive hydrocarbons, their precursors. The most important phytotoxic components produced by these atmospheric photochemical reactions are ozone and peroxyacetyl nitrate. Otherperoxy compounds, aldehydes, ketones, organic and inorganic acids, aerosols, and nitrogen dioxide also are formed. An analysis and evaluation of the available literature was used to characterize the relationships among emissions, ambient concentrations, effects and to identify the important controlling influences on the formation and effects of photochemical oxidants. The ultimate protection of humans, animals, plants, and materials from photochemical oxidant injury requires reduction of the ambient concentration of the particular air pollutant. The available emission estimates for the precursor compounds indicate that, at least for the foreseeable future, humans and the environment will continue to be impacted by photochemical oxidants. The material for this book was selected to provide a basis for preventive measures at the emission source and at the site of impact; also to provide researchers and students with a comprehensive information base.