  There is currently enormous interest in the investigation of how hadron
properties may be altered by immersion in matter. There is strong evidence of a
reduction in the mass of the rho meson from relativistic heavy ion collisions
as well as a hint from a recent experiment on photoproduction in light nuclei.
We briefly review the main theoretical ideas which lead one to expect the mass
of a hadron to change in matter, including the various QCD-based methods,
notably the QCD sum rules, as well as mean-field, quark based models like QMC
and conventional nuclear approaches such as QHD.
