  We review the current status of theories and experiments aiming at an
understanding and a determination of the properties of light vector and scalar
mesons inside strongly interacting hadronic matter. Starting from a discussion
of the relevant symmetries of QCD and their connection with the hadronic
description through QCD sum rules we then discuss hadronic models used to
calculate the in-medium self-energies of hadrons and their spectral functions.
The difficulties to link these calculated properties to actual observables are
emphasized. Finally, we review in detail all the running experiments searching
for in-medium changes of vector and scalar mesons, both with relativistic
heavy-ion reactions as well as with elementary reactions on (cold) nuclei.
Inconsistencies among experimental results are discussed. While almost all
experiments observe a considerable broadening of vector mesons inside the
nuclear medium, no evidence for mass changes is observed in the majority of the
experiments.
