  Experimental results obtained with the HERA collider and recent progress in
their theoretical interpretation are reviewed. After a short introduction to
HERA physics, deep inelastic scattering and photoproduction are discussed as
(virtual) photon-proton scattering. It is shown that the measurement and
theoretical understanding of both photoproduction as well as low-x deep
inelastic scattering are essential for obtaining reliable high energy
extrapolations within hadron-hadron interaction models. Limitations of the
predictive power of hadron interaction models due to the interplay of
perturbative QCD and unitarity effects are discussed.
