  Chiral Lagrangians are used to compute the production rate of photons from
the hadronic phase of relativistic nuclear collisions. Special attention is
paid to the role of the pseudovector a_1 meson. Calculations that include
reactions with strange mesons, hadronic form factors and vector spectral
densities consistent with dilepton production, as well as the emission from a
quark-gluon plasma and primordial nucleon-nucleon collisions, reproduce the
photon spectra measured at the Super Proton Synchrotron (SPS). Predictions for
the Relativistic Heavy Ion Collider (RHIC) are made.
