The SASS (Subsonic Assessment) Ozone and NOx Experiment (SONEX) was an airborne field campaign conducted in October--November 1997 in the vicinity of the North Atlantic Flight Corridor to study the impact of aircraft emissions on NOx and ozone (O3). A fully instrumented NASA DC-8 aircraft was used as the primary SONEX platform. SONEX activities were closely coordinated with the European POLINAT-2 (Pollution from Aircraft Emissions in the North Atlantic Flight Corridor) program, which used a Falcon-20 aircraft. Both campaigns focused on the upper troposphere/lowermost stratosphere (UT/LS) as the region of greatest interest. Specific sampling goals were achieved with the aid of a state-of-the art modeling and meteorological support system, which allowed targeted sampling of air parcels with desired characteristics. A substantial impact of aircraft emissions on NOx, O3, and CN in the UT/LS of the study region is shown to be present. This mission provided direct support for the highly nonlinear nature of the NOx-O3 chemistry. The results are being published in Special Sections of GRL and JGR. This overview provides a context within which these publications can be understood. ¿ 1999 American Geophysical Union