  Lattice quantum chromodynamics (QCD), defined on a discrete space time
lattice, leads to a spectacular non-perturbative prediction of a new state of
matter, called quark-gluon plasma (QGP), at sufficiently high temperatures or
equivalently large energy densities. The experimental programs of CERN, Geneva
and BNL, New York of relativistic heavy ion collisions are expected to produce
such energy densities, thereby providing us a chance to test the above
prediction. After a brief introduction of the necessary theoretical concepts, I
will present a critical review of the experimental results already obtained by
the various experiments in order to examine whether QGP has already been
observed by them.
