Abstract:

The experimental evidence for the phase transition of hadrons to deconfined quarks and gluons in heavy ion reactions observed at the CERN SPS, is critically reviewed. Emphasis is given to threshold behaviour of relevant observables like strangeness and charm production in nuclear reactions occuring at the transition point marked by the critical energy density. Related open questions, relevant to the Relativistic Heavy Ion Collider physics program starting at BNL this summer, will be discussed.