  Recent measurements of diffractive vector-meson production with the
general-purpose detectors H1 and ZEUS in electron-proton interactions with 300
GeV center-of-mass energy at the HERA accelerator complex have stimulated great
interest in the question of whether perturbative QCD (pQCD) can provide an
accurate description of such diffractive processes. The high flux of quasi-real
photons from the 27.5 GeV electron beam has allowed high-statistics studies of
vector-meson photoproduction to be compared to the deeply inelastic processes
at high photon virtuality Q^2 >> \Lambda^2_QCD. Special-purpose electron
detectors at small scattering angle have been used to identify a transition
region, 0.2 < Q^2 < 2 GeV^2, where the pQCD prediction for the dominance of
point-like configurations of qqbar-pairs is verified. Data samples
characterized by high momentum transfer to the 820 GeV initial-state proton, 1
< |t| < 13 GeV^2, have now become available, and the results lend support to
the proposition that pQCD calculations should accurately describe the inelastic
vector-meson production processes in high-|t| photoproduction. Measurements of
the energy dependence and the t-dependence of the elastic cross sections for
rho, omega, phi, and J/psi production have provided tests of the proposal that
the mass (or size) of the vector meson provides a third scaling variable for
studying the transition to the domain of pQCD applicability. A preliminary
result on elastic upsilon production has also been announced by the ZEUS
collaboration. These issues of scale will be discussed in the context of
topical phenomenological models, comparing the HERA results and those from
fixed-target experiments at lower energy to the wealth of calculations
available in the contemporary literature.
