The Kuiper belt contains a vast number of objects in a flattened, ring-like volume beyond the orbit of Neptune. These objects are collisionally processed relics from the accretion disk of the Sun and, as such, they can reveal much about early conditions in the Solar system. At the cryogenic temperatures prevailing beyond Neptune, volatile ices have been able to survive since the formation epoch 4.5 Gyr ago. The Kuiper belt is the source of the Centaurs and the Jupiter-family comets. It is also a local analogue of the dust disks present around some nearby main-sequence stars. While most Kuiper belt objects are small, roughly a dozen known examples have diameters of order 1000 km or more, including Pluto and the recently discovered (and possibly larger) giant Kuiper belt objects 2003 UB313, 2003 EL61 (a binary and a triple system, resp.) and 2005 FY9.