  A status report of utilizing soft electromagnetic radiation (aka thermal
photons and dileptons) in the diagnosis of strongly interacting matter in
ultrarelativistic heavy-ion collisions is given. After briefly elaborating on
relations of the electromagnetic spectral function to chiral symmetry
restoration and the transition from hadron to quark degrees of freedom, various
calculations of electromagnetic emission rates in the hadronic and quark-gluon
plasma phases of QCD matter are discussed. This, in particular, includes
insights from recent thermal lattice QCD computations. Applications to dilepton
and photon spectra in heavy-ion collisions highlight their role as a spectro-,
thermo-, baro- and chrono-meter of extraordinary precision.
