  In principle, quantum chromodynamics provides a fundamental description of
hadronic and nuclear structure and dynamics in terms of their elementary quark
and gluon degrees of freedom. In practice, the direct application of QCD to
reactions involving the structure of hadrons is complex because of the
interplay of nonperturbative effects such as color confinement and multi-quark
coherence. A crucial tool in analyzing such phenomena is the use of
relativistic light-cone quantum mechanics and Fock state methods to provide
tractable and consistent treatments of relativistic many-body systems. We begin
with a brief introduction to light-cone field theory, stressing how it may
allow the derivation of a constituent picture, analogous to the constituent
quark model. The Fock state representation includes all quantum fluctuations of
the hadron wavefunction, including far off-shell configurations such as
intrinsic charm and, in the case of nuclei, hidden color. The Fock state
components of the hadron with small transverse size, which dominate hard
exclusive reactions, have small color dipole moments. Thus QCD predicts minimal
absorptive corrections, i.e., color transparency for quasi-elastic exclusive
reactions in nuclear targets at large momentum transfer. In other applications,
such as the calculation of the axial, magnetic, and quadrupole moments of light
nuclei, the QCD Fock state description provides new insights which go well
beyond traditional hadronic and nuclear physics.
