  We recall the seminal developments in the study of radiation of direct
photons from relativistic heavy ion collisions, which have helped to enhance
the scope of single photons as a probe of the quark gluon plasma considerably.
There is a mounting evidence that in addition to providing information about
the initial temperature of the plasma as envisaged originally, these radiations
measure the momentum anisotropy of the deconfined quarks and gluons, energy
loss of the quarks, the initial spatial asymmetry of the plasma, and the
history of evolution of the system. After a brief description of the
theoretical developments and results for direct photons at SPS energies, we
discuss the expectations and findings at RHIC.
