ULTRARELATIVISTIC NUCLEAR PHYSICS: FROM BECOMING TO BEING

Basic theoretical ideas on a phase transition in heavy ion collisions to a thermalized plasma of free quarks and gluons are outlined. Major experiments are then described which made use of oxygen and sulphur beams with moderate (BNL) or high (CERN) momenta. Representative results pertaining to both average event features and quark-gluon plasma properties are discussed in some detail. This review addresses also interested non-specialists.