  I review recent applications of the open quantum system framework in the
understanding of quarkonium suppression in heavy-ion collisions, which has been
used as a probe of the quark-gluon plasma for decades. The derivation of the
Lindblad equations for quarkonium in both the quantum Brownian motion and the
quantum optical limits and their semiclassical counterparts is explained. The
hierarchy of time scales assumed in the derivation is justified from the
separation of energy scales in nonrelativistic effective field theories of QCD.
Physical implications of the open quantum system approach are also discussed.
Finally, I list some open questions for future studies.
