  We re\,examine the theory of hadronic photon-nucleon interactions at the
quark-gluon level. The possibility of multiple parton collisions in a single
photon-nucleon collision requires an eikonal treatment of the high-energy
scattering process. We give a general formulation of the theory in which the
$\gamma p$ cross section is expressed as a sum over properly eikonalized cross
sections for the interaction of the virtual hadronic components of the photon
with the proton, with each cross section weighted by the probability with which
that component appears in the photon, and then develop a detailed model which
includes contributions from light vector mesons and from excited virtual states
described in a quark-gluon basis. The parton distribution functions which
appear can be related approximately to those in the pion, while a weighted sum
gives the distribution functions for the photon. We use the model to make
improved QCD-based predictions for the total inelastic photon-nucleon and
photon-nucleus cross sections at energies relevant for HERA experiments and
cosmic ray observations. We emphasize the importance in this procedure of
including a soft-scattering background such that the calculated cross sections
join smoothly with low-energy data.
