  We first introduce the conceptual basis of critical behaviour in strongly
interacting matter, with colour deconfinement as QCD analog of the
insulator-conductor transition and chiral symmetry restoration as special case
of the associated shift in the mass of the constituents. Next we summarize
quark-gluon plasma formation in finite temperature lattice QCD. We consider the
underlying symmetries and their spontaneous breaking/restoration in the
transition, as well as the resulting changes in thermodynamic behaviour.
Finally, we turn to the experimental study of strongly interacting matter by
high energy nuclear collisions, using charmonium production to probe the
confinement status of the produced primordial medium. Recent results from Pb-Pb
collisions at CERN may provide first evidence for colour deconfinement.
