The volume gathers the prominent works by participants in the NATO Advanced Study Institute, liThe Comparative Study of the Planets", which was held at Vulcano (Aeolian Islands) from September 14 to September 25, 1981. The book is intended to be a landmark for all those interested in the problems disclosed through the close-up exploration of planets and satellites, either for professional reasons or Simply for scientific knowledge. The topics dealt with concern all methodologies by which the members of the Solar System have been studied and the audience to whom the volume is addressed is, in addition to the experts, mailllly graduate and post-graduate students attending courses in Earth Sciences, Physics and Astro­ nomy. The aim we intend to achieve in editing this volume is to offer to planetologists an overview of the present state of knowledge, using the comparative study of the planets as a basis on which to build the themes treated. We think it is one of the fe, ... publications of this type, which includes all the subjects concerning the study of a planet. We hope we have succeeded in conveying through the book the message which was clearly expressed by the lecturers of the ASI in Vulcano: what counts most at this stage of planetary research is to be able to single out the fundamental problems and choose the appropriate set of data to use in approaching these problems.