  Direct real photons are arguably the most versatile tools to study
relativistic heavy ion collisions. They are produced, by various mechanisms,
during the entire space-time history of the strongly interacting system. Also,
being colorless, most the time they escape without further interaction, ie they
are penetrating probes. This makes them rich in information, but hard to
decypher and interpret. This review presents the experimental and theoretical
developments related to direct real photons since the 1970s, with a special
emphasis on the recently emerged "direct photon puzzle", the simultaneous
presence of large yields and strong azimuthal asymmetries of photons in heavy
ion collisions, an observation that so far eluded full and coherent
explanation.
