  The study of the structure of strongly interacting dense matter via hard jets
is reviewed. High momentum partons produced in hard collisions produce a shower
of gluons prior to undergoing the non-perturbative process of hadronization. In
the presence of a dense medium this shower is modified due to scattering of the
various partons off the constituents in the medium. The modified pattern of the
final detected hadrons is then a probe of the structure of the medium as
perceived by the jet. Starting from the factorization paradigm developed for
the case of particle collisions, we review the basic underlying theory of
medium induced gluon radiation based on perturbative Quantum Chromo Dynamics
(pQCD) and current experimental results from Deep Inelastic Scattering on large
nuclei and high energy heavy-ion collisions, emphasizing how these results
constrain our understanding of energy loss. This review contains introductions
to the theory of radiative energy loss, elastic energy loss, and the
corresponding experimental observables and issues. We close with a discussion
of important calculations and measurements that need to be carried out to
complete the description of jet modification at high energies at future high
energy colliders.
