Freja is a joint Swedish and German satellite, launched on October 6, 1992 and orbiting at 600-1750 kilometre, covering the lower part of the auroral acceleration region. It has been designed to provide high-resolution measurements (both temporal and spatial) of auroral plasma characteristics. The high telemetry rate, together with the 15 Mbyte distributed on-board memories allow Freja to resolve meso and micro-scale phenomena in the 100 metre range for particles and 1-10 metre range for electric and magnetic fields. The UV imager resolves auroral structures of 1 kilometre size at a time resolution of one image every 6 seconds. The novel plasma instruments are orders of magnitude better than any that have gone before. "The Freja Mission" is about the scientific objectives, instruments and platform itself. Detailed descriptions are given of the instrumentation and the first data acquired. This book contains such material in single volume, relating the instruments' design with their in-flight characteristics. It is designed for space engineers and other researchers interested in space science.