Trojans, Centaurs and Trans-Neptunian objects are small dark objects orbiting in the outer Solar System. These objects, still very poorly known, can be considered as fossils of our Solar System, and, as such, they can still contain information about some primordial processes which governed the evolution of the early Solar System. In the next decade, GAIA with its large sky surveys, can allow the discovery of many new objects belonging to these outer solar system populations. In addition, it will provide very precise orbits and visible broad-band colours for many of them.