Ambient air quality varies greatly from place to place at any one time and with the time of day and from year to year at any one place. This chapter looks at measurement networks in the UK and elsewhere that has given people the understanding of the occurrence of air pollutants, and presents some results that demonstrate key aspects of the prevalent conditions, both in the developed and the developing worlds. It shows in some detail at the factors that determine both the short- and long-term airborne concentrations and the rates of dry and wet deposition of gaseous pollutants. The majority of pollutants emitted to air are gaseous, composed predominantly of nitrogen, sulphur or carbon sources. In the winter the relative situation is reversed, because without the solar radiation the Nitric oxide emissions remove the ozone most effectively over mainland Europe, leaving the concentrations higher in the north-west.