This volume tries to summarize the status of observational knowledge of the Kuiper Belt. Its recent discovery has revitalized the astromomical study of the Solar System and is beginning to open new and unexpected windows on the physics of planetesimal accretion. With more and better observational data being obtained at the technological limit of current facilities, a new perception of the relationships that exist among the various classes of small Solar System bodies has emerged. The new observations have also motivated a number of fascinating theoretical studies in Solar System dynamics.