ESA Gaia mission as successor to Hipparcos is planning to map 1% of the stellar population of our galaxy, around one thousand million objects, to micro-arcsecond accuracy. Launched in 2011 the satellite autonomously will follow a pre-defined scanning law which will observe each object about 100 times over the five-year mission. In addition to high precision astrometric information, a prism dispersion assembly will be used to provide multi-band photometry and a spectroscopic instrument provides information for deriving radial velocities. Gaia's focal plane will be the largest ever flown to space comprising a Giga-pixel mosaic of 106 specially designed CCDs operated synchronously in TDI mode. Sophisticated on-board algorithms provide source confirmation and pre-processing of the downlinked data volume of about 100 Terabytes, but only through combining the multiple source information in an iterative on-ground post-processing approach the required precision for the final object catalogue can be achieved. Scientific background as well as technical and organizational realisation of this mission will be presented.