Comet nuclei are the most primitive bodies in the solar system. They have been created far away from the early Sun and their material properties have been altered the least since their formation. Thus, the composition and structure of comet nuclei provide the best information about the chemical and thermodynamic conditions in the nebula from which our solar system formed. In this volume, cometary experts review a broad spectrum of ideas and conclusions based on in situ measurement of Comet Halley and remote sensing observations of the recent bright Comets Hale-Bopp and Hyakutake. The chemical character of comet nuclei suggests many close similarities with the composition of interstellar clouds. It also suggests material mixing from the inner solar nebula and challenges the importance of the accretion shock in the outer nebula. The book is intended to serve as a guide for researchers and graduate students working in the field of planetology and solar system exploration. Several special indexes focus the reader's attention to detailed results and discussions. It concludes with recommendations for laboratory investigations and for advanced modeling of comets, the solar nebula, and the collapse of interstellar clouds.