Hadrons in dense matter

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. (orig.)