  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 $\eta$- and
vector-meson production. We show that photoproduction of $\eta$'s can yield
essential information on in-medium properties of the $S_{11}(1535)$ resonance
while the $\phi$ meson properties will probably not be accessible through the
$K^+K^-$ decay channel. However, for $\omega$ mesons the $\pi^0\gamma$ 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.
