POLAR is one of four spacecraft in the Global Geospace Science (GGS) program. These are among the six spacecraft in the International Solar Terrestrial Physics (ISTP) program. POLAR provides multi-wavelength imaging of the aurora, measuring plasma entry into the polar magnetosphere and geomagnetic tail, the flow of plasmas to and from the ionosphere, and the deposition of particle energy in the ionosphere and upper atmosphere. POLAR has on-board propulsion systems and a design lifetime of three to five years, with redundant subsystems. POLAR is cylindrical, approximately 2.8 m in diameter by 1.25 m high (plus 1.25 m for its two despun platforms), with body-mounted solar cells, weighs 1250 kg and uses 333 W of power. The spin rate is 10 rpm around an axis approximately normal to the orbital plane. It has long wire spin-plane antennas, inertial booms, and spin-plane appendages to support sensors. POLAR has two despun gimbaled instrument platforms, and booms are deployed along both Z axes. Data are stored using on-board tape recorders and are relayed to the Deep Space Network at 600 kbps maximum (250 kbps nominal) although the average real-time data rate for POLAR is 41.6 kbps. POLAR has a 22.6-h polar orbit (90 deg inclination), with perigee and apogee of 11,500 and 57,000 km. Polar was launched to observe the polar magnetosphere and, as its orbit has precessed with time, has observed the equatorial inner magnetosphere and is now carrying out an extended period of southern hemisphere coverage. Details on the POLAR mission and instrumentation are provided in Space Science Reviews (Vol. 71, Nos. 1-4, 1995) and reprinted in The Global Geospace Mission, edited by C. T. Russell (Kluwer, 1995).