  Strongly interacting matter as described by the thermodynamics of QCD
undergoes a phase transition, from a low temperature hadronic medium to a high
temperature quark-gluon plasma state. In the early universe this transition
occurred during the early microsecond era. It can be investigated in the
laboratory, in collisions of nuclei at relativistic energy, which create
"fireballs" of sufficient energy density to cross the QCD Phase boundary. We
describe 3 decades of work at CERN, devoted to the study of the QCD plasma and
the phase transition. From modest beginnings at the SPS, ultra-relativistic
heavy ion physics has evolved today into a central pillar of contemporary
nuclear physics and forms a significant part of the LHC program.
