Trans-neptunian objects (TNOs) are believed to be pristine remnants of planets formation, providing constrains on the early stages of the solar system evolution. The knowledge of this population composition and dynamics provides constrains on the formation processes of the early solar nebula, as well as formation processes of other planetary systems around young stars. Nonetheless, because of their great heliocentric distance, and their resulting faintness, all studies are very challenging. 
More than a thousand objects have been detected though, and orbits have been determined for most of them. The resulting dynamical structure is complex and not fully understood: we divide the trans-neptunian region into the Kuiper Belt –itself divided into resonant objects and the classical belt–, the scattered disk and detached objects. 
TNOs physical properties remain poorly known, but we can get constrains on their size, shape, mass, albedo, density or color using different observational methods. Composition is the most difficult property to access though. Only few spectra are available, and they show nevertheless features due to the presence of diverses ices. All those properties result from the competition of several processes that will be discussed. 
The more we learn about TNOs, the more the picture seems complicated. Ultimately, extremely large telescopes, new satellites such as Herschel and space missions like New Horizons will be of great help, since a better understanding of their properties and evolution is critical to improve solar and extra-solar systems formation models.