Lawrence Berkeley National Laboratory

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\approx 0.5 GeV/fm. From the recent PHENIX data of high p_T pi_0 spectra in centra l 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.