  Quarkonium has long been proposed as one of the golden probes to identify the
phase transition from confined hadronic matter to the deconfined quark-gluon
plasma in heavy-ion collisions. Since then, we have achieved a better
understanding, not only about the propagation of quarkonium in-medium, but also
about its production. Recent theoretical developments in our comprehension of
the quarkonium production mechanism in proton-proton collisions and on the
propagation of quarkonia in proton-nucleus and nucleus-nucleus collisions are
reviewed and discussed.
