ar X iv : a st ro - p h / 98 11 22 5 v 1 1 3 N ov 1 99 8 1

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.