The quark gluon plasma, a plasma of strongly-correlated quarks and gluons, is created in the laboratory by colliding heavy ions at relativistic speeds. These collisions generate the high energy densities and temperatures needed to free the quarks and gluons bound inside nuclei. The resulting QGP is both a medium exhibiting emergent phenomena and a high energy density environment in which quantum chromodynamics, the fundamental theory describing quark and gluon interactions, is investigated. Direct photons are clean probes of nuclear collision systems because they are produced throughout the collision and do not re-interact with the system after emission. In this talk, I will present current and future direct photon measurements at RHIC and the LHC, highlighting how they are used to understand key properties of the QGP such as temperature.