When nuclei collide at beam energies from several tens of MeV to several GeV per nucleon considerable disorder is generated. Nuclear fragments ranging from nucleons all the way up in mass to the target and projectile nuclei themselves have been observed experimentally. Theoretical models for the dynamics of the formation and emission of these clusters of nucleons are reviewed. Most of the models, but not all, are statistical in origin, following from the assumption that the phase space available for cluster formation and emission is the dominant factor. The entropy generated during the collision may be studied in diverse dynamical models, such as intranuclear cascade and nuclear fluid dynamics. The entropy of the system may be estimated from the measured abundances of nuclear clusters, thus providing information on the properties of hot and dense nuclear matter. Critical analysis of both conventional and exotic interpretations of the data are given.
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We gratefully acknowledge all of our colleagues with whom we have had discussions over the years. In particular we would like to point to the role of the Nuclear Science Division of Lawrence Berkeley Laboratory for its part in nurturing the development of this field and which has been the mecca of relativistic heavy ion physics since its inception more than ten years ago. This report was supported by the U.S. Department of Energy under contract DOE/DE-ACO2-79ER-10364.