  Transversity observables, such as the T-odd single-spin asymmetry measured in
deep inelastic lepton scattering on polarized protons, and the distributions
which are measured in deeply virtual Compton scattering provide important
constraints on the fundamental quark and gluon structure of the proton. In this
talk I discuss the challenge of computing these observables from first
principles; i.e., quantum chromodynamics, itself. A key step is the
determination of the frame-independent light-front wavefunctions (LFWFs) of
hadrons -- the QCD eigensolutions which are analogs of the Schrodinger
wavefunctions of atomic physics. The lensing effects of initial-state and
final-state interactions, acting on LFWFs with different orbital angular
momentum, lead to the T-odd transversity observables such as the Sivers,
Collins, and Boer-Mulders distributions. The lensing effect also leads to
leading-twist phenomena which break leading-twist factorization, such as the
breakdown of the Lam-Tung relation in Drell-Yan reactions. A similar
rescattering mechanism also leads to diffractive deep inelastic scattering, as
well as nuclear shadowing and non-universal antishadowing. It is thus important
to distinguish "static" structure functions, the probability distributions
computed from the target hadron's light-front wavefunctions, versus "dynamical"
structure functions which include the effects of initial- and final-state
rescattering. I also discuss related effects, such as the J=0 fixed pole
contribution which appears in the real part of the virtual Compton amplitude.
AdS/QCD, together with "Light-Front Holography", provides a simple
Lorentz-invariant color-confining approximation to QCD which is successful in
accounting for light-quark meson and baryon spectroscopy as well as hadronic
LFWFs.
