  Medium modification of jet fragmentation functions and parton energy loss in
cold and hot matter are reviewed. The predicted nuclear modification of the jet
fragmentation function agrees well with the recent HERMES data with a resultant
energy loss dE/dx approximately 0.5 GeV/fm. From the the recent PHENIX data of
high p_T pi_0 spectra in central Au+Au collisions at sqrt(s)=130 GeV, one
extracts an energy loss for a 10 GeV parton that is equivalent to dE/dx=7.3
GeV/fm in a static medium with the same gluon density as in the initial stage
of the collision at tau_0=0.2 fm/c. Constraints on jet quenching by the central
rapidity density of charged hadrons is also discussed.
