  Central nuclear collisions at energies far above 1 GeV/nucleon may provide
for conditions, where the transition from highly excited hadronic matter into
quark matter or quark-gluon plasma can be probed. Here I review our current
understanding of the physical properties of a quark-gluon plasma and review
ideas about the nature of, and signals for, the deconfinement transition. I
also give a detailed presentation of recent progress in the treatment of the
formation of a thermalized state at the parton level.
