  Due to their penetrating nature, dileptons are a valuable probe for the
properties of the hot and/or dense medium created in relativistic heavy-ion
collisions. Dilepton invariant-mass spectra provide direct access to the
properties of the electromagnetic current-correlation function in strongly
interacting matter. In this paper an overview is given of our current
theoretical understanding of the dilepton phenomenology in comparison to recent
data in heavy-ion collisions at the CERN SPS.
