A comprehensive assessment of the impact of aircraft emissions has concluded that these emissions are small in comparison to other man-made emissions but could have a significant effect on atmospheric chemistry and cloud cover, with possible implications for the ozone layer and climate. The review was carried out by leading European researchers, under the auspices of the EU's Environment and Climate research programme, and the results have just been published in the scientific journal "Atmospheric Environment". The findings of the review rely heavily on the results of European and national research projects, such as AERONOX, POLINAT, MOZAIC, AEROCHEM, AEROCONTRAIL, and LOWNOX projects supported by the Environment and Climate programme and the Industrial and Materials Technologies (BRITE-EURAM) programme, as well as national projects. The authors conclude that, firstly, the warming (climate) effect associated with the ozone increase from NOx emissions is comparable to the warming effect of CO2 emitted by aircraft, secondly, the warming effect of the changes in cloudiness through contrail formation appears also to be of the same magnitude as the warming effect of CO2, and thirdly, the development of a fleet of supersonic aircraft flying at high altitudes could perturb the ozone layer in the stratosphere. A number of gaps in existing knowledge have been revealed in the review, in particular the lack of understanding of the region where aircraft fly, which straddles the boundary between troposphere and stratosphere. In addition, the need for study of the effect of aircraft emissions on the abundance of particles providing the surface for complex heterogeneous reactions was also highlighted. Work is continuing on these issues through the Environment and Climate programme and the Coordination of Research for the Study of Aircraft impact on the Environment (CORSAIRE) initiative. Research will continue under the upcoming Fifth RTD Framework Programme, in particular, under the Key Actions on "Global change, climate and biodiversity" and "New perspectives in aeronautics". At international level, European scientists will also continue to participate in the "Aviation and Global Atmosphere" report, currently being carried out by the Intergovernmental Panel on Climate Change (IPCC).