This book deals with nucleus-nucleus interactions at energies ranging from about 1GeV per nucleon to the highest energies available from present accelerators and cosmic rays. A brief discussion about the relevant theoretical models is given, with special emphasis on the formation of quark-gluon plasma. Some of the experimental techniques of relativistic heavy ion physics - accelerators and detectors, are also described. The main part of the text concerns experimental results pertaining to various aspects of high energy heavy ion collisions: fragmentation processes, reaction cross sections, secondary particle multiplicities, particle spectra, strangeness and heavy flavour production, particle correlations, and global variables such as transverse energy, entropy and energy density. In addition, some open issues and the prospects of this fascinating and rapidly evolving field are discussed. An exhaustive list of references is included.