  We discuss some of the aspects of the physics of relativistic nuclear
collisions, in particular those having to do with the observation of
electromagnetic radiation. We concentrate on what such measurements tell us
about the local, in-medium properties of the environment from which they
emerge. The contribution from different sources are considered: that from the
partonic sector of QCD, and that from the confined hadronic phase.
Specifically, we discuss the observation of real photons and of lepton pairs at
the SPS and at RHIC, and make predictions for the LHC. The role of jets is
discussed.
