  Measurements of the total cross section for real and virtual photons on
protons at center-of-mass energies in the range from 20 GeV to 270 GeV with
photon virtualities up to 5000 GeV^2 are presented. For real photons this cross
section can be described by Regge-motivated models while for virtual photons
perturbative QCD can be applied. The measurements of the two HERA collider
experiments ZEUS and H1 open the possibility to investigate the interplay
between the two theoretical approaches in the transition region as well as the
high-energy behavior of the cross sections. The results of total cross section
measurements are discussed in the above context.
