  We discuss how the dijet azimuthal correlations in DIS and real
photoproduction at HERA probe the differential (unintegrated) gluon
distribution in the proton. We find a strong dependence of the azimuthal
correlation pattern on Bjorken-$x$, photon virtuality and the cut on the jet
transverse momenta. A rise of the azimuthal decorrelations is observed with
decreasing Bjorken-x due to the interplay of perturbative and nonperturbative
effects. We predict a strong rise of the same-side jet rate with photon energy
for real photoproduction. We discuss conditions for the correlation function to
be dominated by hard perturbative gluons and ways of constraining the size of
the nonperturbative soft component. We make some predictions for the THERA
energy range. The analysis of the energy dependence of the isolated jet and
two-jet cross sections in photoproduction would be a new way to study the not
yet well constrained unintegrated gluon distributions and to explore the onset
of the pQCD regime.
