  QCD jets, produced copiously in heavy-ion collisions at LHC and also at RHIC,
serve as probes of the dynamics of the quark-gluon plasma (QGP). Jet
fragmentation in the medium is interesting in its own right and, in order to
extract pertinent information about the QGP, it has to be well understood. We
present a brief overview of the physics involved and argue that jet
substructure observables provide new opportunities for understanding the nature
of the modifications.
