  Relativistic heavy ion physics studies the phenomena that occur when a very
large (in units of QCD scale $\Lambda_{\rm QCD}$) amount of energy is deposited
into a large (in units of $\Lambda^{-3}_{\rm QCD}$) volume, creating an
extended in space and time domain with an energy density that is large in units
of $\Lambda^{4}_{\rm QCD}$. This includes the mechanism by which the energy is
deposited (likely a transformation of the colliding Lorentz-contracted "gluon
walls" into the strong longitudinal color fields); approach to thermalization;
and the static and dynamical properties of the created quark-gluon plasma. Of
particular interest is the fate of symmetries (e.g. chiral $SU_L(3) \times
SU_R(3)$, scale, and discrete ${\cal P}$ and ${\cal CP}$ invariances) in hot
and dense QCD matter. At present, the program at RHIC has entered a stage where
new discoveries are enabled by high precision of the measurements; moreover, an
array of new capabilities will soon be available due to the numerous and
significant upgrades. Very importantly, we will soon have access to
unprecedented energies of colliding ions at the LHC. In addition, future RHIC
runs at low energies, FAIR at GSI and NICA at JINR will make possible the
studies of QCD matter at high baryon density. I will describe the current
status of theoretical knowledge about hot QCD, and the ways in which it may be
expected to improve in the near future.
