The 1984 Cargese Advanced Study Institute was devoted to the study of nuclear heavy ion collisions at medium and ultrarelativis tic energies. The origin of this meeting goes back to 1982 when the organizers met at the GANIL laboratory in Caen, France which had just started accelerating argon ions at 44 MeV per nucleon. We then realized that 1984 should be the appropriate time to review the first results obtained with such new kinds of facilities. The material contained in this volume, presenting many beautiful re sults on nuclei at high excitation, fully confirms this point. Many stimulating exchanges between experts in rather diffe rent fields already took place during the school and we hope that this cross fertilization will lead to further developments. About half of the present volume is also devoted to the field of relativistic heavy ion collisions, which is now expanding rapidly. As an illustration, let us recall that the construction of a 30 on 30 GeV per nucleon collider at Brookhaven has been recognized last year as one cf the major priorities by the US Nuclear Science Advisory Committee. We would like to express our gratitude to NATO for its ge nerous financial support which made this institute possible. We also wish to thank the Institut de Physique Nucleaire et de Physique des Particules (France), the Commissariat a l'energie atomique (France) and The National Science Foundation (USA) for the attribution of travel grants.