ESA's Gaia mission is the next European breakthrough in astrophysics, a cornerstone mission scheduled for launch in November 2013 which is designed to produce the most accurate 3D map of the Milky Way to date. Over the course of its five-year mission Gaia will measure positions, parallaxes, and proper motions for every object in the sky brighter than magnitude 20, amounting to about 1 billion stars, galaxies, quasars and solar system objects.It will achieve an astrometric accuracy of 12-25 micro-arcsecond, depending on colour, at 15th magnitude and 100-300 micro-arcsecond at 20th magnitude. Multi-colour photometry will be obtained for all objects by means of low-resolution spectrophotometry between 330 and 1000nm. In addition radial velocities with a precision of 1-15 km/s will be measured for all objects to 17th magnitude, thus complementing the astrometry to provide full six-dimensional phase space information for the brighter sources.The number of stars in the Gaia catalogue is estimated to be about 700000 to 10th magnitude, 48 million to 15th magnitude and 1.1 billion to 20th magnitude. About 60 million stars are expected to be seen as binary or multiple systems by Gaia, among which about 1 to 10 million eclipsing binaries. In addition the catalogue will contain astrometry and photometry for about 300000 solar system bodies, 500000 quasars, and some 1 to 10 million galaxies. The sky survey by Gaia will also produce the most accurate optical all-sky map ever, with an angular resolution comparable to that of the Hubble Space Telescope. The range of science questions that can be addressed with such a data set is immense and Gaia will revolutionize studies of the Milky Way, stars, exoplanets, the transient sky, and our solar system.The on-ground data processing for Gaia is a very large and highly complex task, linking all astrometric, photometric and radial velocity measurements into a large iterative solution, which ultimately leads the published Gaia catalogue and archive. The multitude of data processing tasks will be undertaken by the scientific community in Europe which has organized itself into the Gaia Data Processing and Analysis Consortium (DPAC). The data processing activities will be structured around nine 'coordination units' (CUs) and six data processing centres. Each CU is responsible for delivering a specific part of the overall data processing system for Gaia. The processing systems will then run at the various data processing centres. The DPAC consists of about 450 persons, spread over academic institutes and space agencies throughout Europe and beyond, who contribute to writing the millions of lines of code needed for the data processing and to subsequent operation of the software systems and the validation of the resulting output.