Nuclear collisions as seen through photons

The energy spectrum of photons is understood well in ultrarelativistic collisions of protons or of nuclei, as long as the photons' energy reaches the multiple GeV range. For GeV-energy photons, however, considerable differences appear between the collisions of protons and of nuclei. In the heavy-ion case, the production of GeV-energy photons appears to be dominated by electromagnetic radiation from the plasma of deconfined nuclear matter produced in the collisions. I will discuss the status of theoretical calculations of the photon energy spectrum in proton-proton and heavy-ion collisions, reviewing the current agreement with experimental data, as well as recent developments.