  A model combining perturbative and non-perturbative QCD is developed to
compute high-energy reactions of hadrons and photons and to investigate
saturation effects that manifest the S-matrix unitarity. Following a functional
integral approach, the S-matrix factorizes into light-cone wave functions and
the universal amplitude for the scattering of two color-dipoles which are
represented by Wegner-Wilson loops. In the framework of the non-perturbative
stochastic vacuum model of QCD supplemented by perturbative gluon exchange, the
loop-loop correlation is calculated and related to lattice QCD investigations.
With a universal energy dependence motivated by the two-pomeron (soft + hard)
picture that respects the unitarity condition in impact parameter space, a
unified description of pp, pip, Kp, gamma* p, and gamma gamma reactions is
achieved in good agreement with experimental data for cross sections, slope
parameters, and structure functions. Impact parameter profiles for pp and
longitudinal gamma* p reactions and the gluon distribution of the proton
xG(x,Q^2,b) are calculated and found to saturate in accordance with S-matrix
unitarity. The c.m. energies and Bjorken x at which saturation sets in are
determined.
