Passenger air traffic in the world has doubled over the last 15 years. Over the last few years, European air traffic movements have increased by between 4% and 6% per annum and current forecasts indicate that European air traffic movements are likely to double by 2015 compared with 1998 figures. Most of the aviation fuel is consumed in the cruising phase, which for the majority of flights takes place at around 10 km altitude. Hence most pollutants emitted are released in this part of the atmosphere. The contribution of gaseous and particulate emissions by aviation is small in comparison to other global sources. However, as these emissions occur directly in the upper troposphere or lower stratosphere, those processes which would otherwise remove the gases and particles from the environment are largely ineffective. This means that the residence time of pollutants in the upper atmosphere is much greater than that of pollutants emitted near the ground. The main contribution of commercial air traffic to changes in the atmosphere’s composition takes place at altitudes between 9 and 13 kilometres. These changes may have impacts on the climate.