  Modifications in the production pattern of heavy quark bound states have long
been considered to provide sensitive signatures for the thermal properties of
dense matter created in heavy ion collisions. The original concept of Matsui
and Satz for quarkonium suppression as signature for deconfinement in heavy ion
collisions has been challenged recently through lattice studies of spectral
functions, which indicate the persistence of heavy quark bound states at
temperatures well above the transition, as well as through the refined analysis
of hadronization and recombination models, which take into account the thermal
evolution of the medium generated in a heavy ion collision. We will review here
recent developments on these topics.
