  At high temperatures, strongly interacting matter becomes a plasma of
deconfined quarks and gluons. In statistical QCD, deconfinement and the
properties of the resulting quark-gluon plasma can be investigated by studying
the in-medium behaviour of heavy quark bound states. In high energy nuclear
interactions, quarkonia probe different aspects of the medium formed in the
collision. We survey the results of recent charmonium production studies in SPS
and RHIC experiments.
