  Parallel to advances in laser cooling of atoms and ions in dilute gas phase,
which has progressed immensely, resulting in physics Nobel prizes in 1997 and
2001, major progress has recently been made in laser cooling of solids. I
compare the physical nature of the laser cooling of atoms and ions with that of
the laser cooling of solids. I point out all advantages of this new and very
promising area of laser physics. Laser cooling of solids (optical
refrigeration) at the present time can be largely divided into three areas:
laser cooling in ion doped glasses or crystals, laser cooling in semiconductors
(bulk or confined, like quantum-well structures), and radiation-balanced
lasers. All three areas, which are equally important and promising, are covered
in the paper.
