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.

The study of the Solar system, particularly of its newly discovered outer parts, is one of the hottest topics in modern astrophysics with great potential for revealing fundamental clues about the origin of planets and even the emergence of life. The three lecturers of the 35th Saas-Fee Advanced Course, which have been updated and collected in this volume, cover the field from observational, theoretical and numerical perspectives.