Quarkonium propagation in the quark-gluon plasma

In relativistic heavy ion collisions at RHIC and the LHC, a quark-gluon plasma (QGP) is created for a short duration of about 10 fm/c. Quarkonia (bound states of cc and b (b) over bar) are sensitive probes of this phase on length scales comparable to the size of the bound states which are less than 1 fm. Observations of quarkonia in these collisions provide us with a lot of information about how the presence of a QGP affects various quarkonium states. This has motivated the development of the theory of heavy quarks and their bound states in a thermal medium, and its application to the phenomenology of quarkonia in heavy ion collisions. We review some of these developments here.