We present the science objectives and instrument development of the Polar Outflow Probe (POP), a micro-satellite mission led by the Institute for Space Research at the University of Calgary. The POP mission was selected in the 1996 CSA Small Payloads Program Announcement of Opportunity competition for Phase A Study, and was subsequently selected for development as a "hot-standby" mission. The science objective of POP is to explore plasma and atmospheric outflow processes in the polar ionosphere and upper atmosphere between 300 and 1500 km, where the neutral atmospheric population is largely unexplored and the plasma population is under-explored. The science objectives of POP are to quantify the micro-scale characteristics of plasma outflow in the polar ionosphere and probe-related micro- and meso-scale plasma processes at unprecedented resolution, and explore the occurrence morphology of neutral escape in the upper atmosphere. The POP mission entails a single micro-satellite in elliptic high-inclination orbit: a 70 kg, 0.5 × 0.5 × 0.5 m3 bus including a 25 kg science instrument payload, a nominal perigee of 300500 km, an apogee of 10001500 km, and an inclination of 65°110°. To achieve the elliptical high-inclination orbit, it is planned to launch POP as a secondary payload into a polar low-Earth orbit (LEO) and then place it in its final orbit using an on-board booster motor. The science instrument payload will consist of five science instruments. These include an imaging and rapid-stepping ion mass/energy/angle spectrometer, a time-of-flight neutral mass and velocity spectrometer, a suprathermal electron imager, a fast auroral imager, and a radio receiver instrument. The particle instruments will be capable of measuring ion, electron, and neutral velocities at unprecedented temporal-spatial resolutions from an orbiting platform. In addition, the payload will include a fluxgate magnetometer and a differential GPS receiver, which will be used for spacecraft attitude and position determination, and scientific measurements of magnetic field perturbation and ionospheric irregularities. The fast auroral imager and the neutral mass spectrometer are both new instruments that are being developed at the University of Calgary and the Institute of Space and Astronautical Science (ISAS), respectively. The other instruments are improved or modified versions of flight-proven instruments in previous Canadian missions.