  A modification of the internal structure of jets is expected due to the
production of a dense QCD medium, the Quark Gluon Plasma, in heavy-ion
collisions. We discuss some aspects of jet reconstruction in p+p and A+A
collisions and emphasize the dramatically increased contribution of the
underlying event in nucleus-nucleus collisions as compared with the vacuum
case. We conclude with its consequences on the full jet spectrum and
fragmentation function extraction at LHC.
