We first briefly review the theoretical basis for calculations of changes of hadronic properties in dense nuclear matter. These changes have usually been investigated by means of relativistic heavy-ion reactions. Here we discuss that observable consequences of such changes can also be seen in more elementary reactions on nuclei. Particular emphasis is put on a discussion of actual observables in photonuclear reactions; we discuss in detail $η$- and vector-meson production. We show that photoproduction of $η$'s can yield essential information on in-medium properties of the $S_{11}(1535)$ resonance while the $ϕ$ meson properties will probably not be accessible through the $K^+K^-$ decay channel. However, for $ω$ mesons the $π^0γ$ decay channel, due to its reduced final state interaction, looks more promising in this respect. Completely free of final state interactions is dilepton production in the few GeV range. We show that the sensitivity of this decay channel to changes of hadronic properties in medium in photonuclear reactions on nuclei is as large as in ultrarelativistic heavy ion collisions. Finally we discuss that hadron production in nuclei at 10 -- 20 GeV photon energies can give important information on the hadronization process.

Invited Lecture by U. Mosel at Erice International School on Nuclear Physics 2003