  Hard hadronic exclusive processes are now at the forefront of QCD studies,
particularly because of their role in the interpretation of exclusive hadronic
B decays. Perturbative QCD and its factorization properties at high momentum
transfer provide an essential guide to the phenomenology of exclusive
amplitudes at large momentum transfer--the leading power fall-off of form
factors and fixed-angle cross sections, the dominant helicity structures, and
their color transparency properties. The hard scattering subprocess amplitude
T_H controlling the leading-twist amplitude is evaluated in the perturbative
domain where the propagator virtualities are above the separation scale. A
critical question is the momentum transfer required such that leading-twist
perturbative QCD contributions dominate. I review some of the contentious
theoretical issues and empirical challenges to Perturbative QCD based analyses,
such as the magnitude of the leading-twist contributions, the role of soft and
higher twist QCD mechanisms, the effects of non-zero orbital angular momentum,
the possibility of single-spin asymmetries in deeply virtual Compton
scattering, the role of hidden color in nuclear wavefunctions, the behavior of
the ratio of Pauli and Dirac nucleon form factors, the apparent breakdown of
color transparency in quasi-elastic proton-proton scattering, and the
measurement of hadron and photon wavefunctions in diffractive dijet production.
