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...