  Lattice QCD predicts a phase transition between hadronic matter and a system
of deconfined quarks and gluons (the Quark Gluon Plasma) at high energy
densities. Recent results from the Brookhaven Relativistic Heavy Ion Collider
(RHIC) dedicated to the study of QCD at extreme densities will be discussed and
compared to measurements obtained at the CERN Super Proton Synchrotron (SPS).
