This report summarises the results of a workshop on the physicalproperties of Kuiper belt objects (KBOs), held at the Observatoire deParis (Meudon, France) on 2001 June 5–8. In the 10 years since the discovery ofthe first KBO, about 500 have been identified; some of these are binaryobjects. Photometric, spectroscopic, and radiometric studies have begun toreveal the basic physical characteristics of these objects. Their diversecolours and spectral properties are clues to their origin and evolution,and the role they may have played in bringing volatile materials to theinner planets.