The physics of ultra‐relativistic heavy ions is presented, with particular emphasis on the basis concept used in interpreting the data and on the principal problems encountered in this new and rapidly evolving field. A number of selected results from experiments at CERN and BNL are reviewed and compared qualitatively with models based on hadronic scenarios as well as models including quark‐gluon plasma (QGP) formation. No unambiguous evidence for QGP formation has emerged from the data so far, but a number of large and significant effects have been observed which clearly show that nucleus‐nucleus collisions cannot be described as a straightforward superposition of independent nucleon‐nucleon reactions. Results from a number of different observables (pTdistributions, strangeness abundances,J/&psgr; suppression) indicate the existence of an extended and strongly interacting system. The first ‘‘exploratory’’phase of experimentation seems therefore to confirm that ultra‐relativistic heavy‐ion collisions are a promising tool to study the properties of bulk hadronic matter under extreme conditions.