  We review the properties of energetic parton propagation in hot or cold QCD
matter, as obtained in recent works. Advances in understanding the energy loss
- collisional and radiative - are summarized, with emphasis on the latter: it
features very interesting properties which may help to detect the quark-gluon
plasma produced in heavy ion collisions. We describe two different theoretical
approaches, which lead to the same radiated gluon energy spectrum. The case of
a longitudinally expanding QCD plasma is investigated. The energy lost by a jet
with given opening angle is calculated in view of making predictions for the
suppression (quenching) of hard jet production. Phenomenological implications
for the difference between hot and cold matter are discussed. Numerical
estimates of the loss suggest that it may be significantly enhanced in hot
compared to cold matter.
