The Airborne Polar Experiment-Polar Ozone, Leewaves, Chemistry and Transport (APE-POLECAT) mission took place between December 19, 1996, and January 16, 1997. APE-POLECAT comprised the inaugural mission of the high-altitude research aircraft, the M-55 Geophysica, flights by the DLR Falcon, measurements from a number of Arctic ground stations, and atmospheric modeling. Both aircraft flew out of Rovaniemi in Finland. The Geophysica was equipped with a payload designed to probe the chemistry and microphysics of polar stratospheric clouds (PSCs) at, or above, the aircraft altitude (up to 20 km geometric altitude). The Deutsches Zentrum fur Luft und Raumfahrt (DLR) Falcon was fitted with an aerosol lidar called OLEX, which looked upward. Ground-based measurements included aerosol lidar, meteorological sondes, and ozone sondes, from both sides of the Scandinavian Mountains, and from the southern Arctic Ocean. The original primary aim of the mission, to study PSC processes in situ, was modified in the light of unfavorable meteorological conditions. Flights concentrated on studies of transport and chemistry around the polar vortex, and on remote sensing of very high, mountain-wave--induced, PSCs. Here we report the objectives and rational of the mission, provide basic descriptions of the conditions of the stratosphere at the time of each flight, and give a summary of the measurements made. ¿ 1999 American Geophysical Union