  In relativistic heavy ion collisions at RHIC and the LHC, a quark gluon
plasma (QGP) is created for a short duration of about 10fm/c. Quarkonia (bound
states of $c\bar{c}$ and $b\bar{b}$) are sensitive probes of this phase on
length scales comparable to the size of the bound states which are less than
1fm. 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.
