MOZAIC was initiated in 1993 by European scientists, aircraft manufacturers and airlines to better understand the natural variability of the chemical composition of the atmosphere and how it is changing under the influence of human activity, with particular interest in the effects of aircraft. MOZAIC consists of automatic and regular measurements of reactive gases by five long range passenger airliners. A large database of measurements (about 30,000 flights since 1994) allows studies of chemical and physical processes in the atmosphere, validations of global chemistry transport models and satellite retrievals. MOZAIC data provide detailed  climatologies of trace gases at 9-12 km where subsonic aircraft emit most of their exhaust and which is a very critical domain (e.g. radiatively and Stratosphere-Troposphere exchanges) still imperfectly described in existing models. MOZAIC data also provide frequent vertical profiles over a large number of airports (Frankfurt, Paris, Vienna, New-York, Atlanta, Tokyo, Beijing, Sao Paulo, Johannesburg, ...).