  In nucleus-nucleus collisions at ultra-relativistic energies matter is formed
with initial energy density significantly exceeding the critical energy density
for the transition from hadronic to partonic matter. We will review the
experimental evidence for this new form of matter - the Quark-Gluon Plasma -
from recent experiments at the SPS and RHIC with emphasis on collective
behavior, thermalization, and its opacity for fast partons. We will further
show that one can determine from the data a fundamental QCD parameter, the
critical temperature for the QCD phase transition.
