  A fundamental understanding of quantum chromodynamics, particularly at the
amplitude level, is essential for progress in high energy physics. For example,
the measurement and interpretation of the basic parameters of the electroweak
theory and CP violation depends on an understanding of the dynamics and phase
structure of exclusive B-meson decay amplitudes. In this review, I discuss a
number of ways in which the required hadron wavefunctions can be measured (such
as two-photon reactions and diffractive dissociation) or calculated from first
principles. An important tool for describing relativistic composite systems in
quantum field theory is the light-front Fock expansion, which encodes the
properties of a hadrons in terms of a set of frame-independent n-particle
wavefunctions. Evolution in light-cone time allows the construction of an
"event amplitude generator" in which only non-ghost physical degrees of freedom
and integration over physical phase appear. The diffractive dissociation of a
hadron at high energies, by either Coulomb or Pomeron exchange, has a natural
description in QCD as the materialization of the projectile's light-cone
wavefunctions; in particular, the diffractive dissociation of a meson, baryon,
or photon into high transverse momentum jets measures the shape and other
features of the projectile's distribution amplitude. I also review recent work
which shows that the structure functions measured in deep inelastic lepton
scattering are affected by final-state rescattering, thus modifying their
connection with the light-cone probability distributions. In particular, the
shadowing of nuclear structure functions is due to destructive interference
effects from leading-twist diffraction of the virtual photon, physics not
included in the nuclear light-cone wavefunctions.
