  Direct photons are a powerful tool to study the hot and dense matter created
in heavy-ion collisions at RHIC, since they are created in the different stages
of the collision. Since they do not interact via the strong force, they can
travel through the hot and dense matter mostly unaffected. The PHENIX
experiment has measured direct photons using different methods, over a broad
range of transverse momentum (\pt), in different collision systems, and at
different energies. These measurements help interpreting the measurement of
hadrons as well as understanding the temperature of the created quark-gluon
plasma (QGP). The azimuthal anisotropy of direct photons may shed light on the
thermalization time of the medium. Using direct photons to tag jets is a
crucial tool to understand the energy loss of scattered partons in the medium.
