Hadronic photo-absorption

The new theoretical and experimental results concerning the hadronic absorption of real and virtual photons by nucleons and nuclei at energies above the meson production threshold are discussed. Magnitudes of the total photoabsorption cross sections on nucleons σsub(t)(γN) and nuclei σsub(t)(γA) contain the data on new particles, on the role of pions in the nuclear matter, on the hadronic structure of a photon, and the relevant data serve as an important criteria in testing the quark models, the parton models and the vector dominance models. The necessity in measurements of σsub(t)(γN) at energies above 30 GeV and in studies of the shadow effects in σsub(t)(γA) on the degree of virtuality of photons for further development of these models is emphasized. The systematic studies of the photoabsorption by nuclei in the region of the excitation of nucleon resonances can reveal up important information on mesonic exchange currents