  Recent theoretical studies have provided new insight into the intrinsic
transverse momentum distributions of valence and sea quarks in the nucleon at a
low scale. The valence quark transverse momentum distributions (q - qbar) are
governed by the nucleon's inverse hadronic size R^{-1} ~ 0.2 GeV and drop
steeply at large p_T. The sea quark distributions (qbar) are in large part
generated by non-perturbative chiral-symmetry-breaking interactions and extend
up to the scale rho^{-1} ~ 0.6 GeV. These findings have many implications for
modeling the initial conditions of perturbative QCD evolution of TMD
distributions (starting scale, shape of p_T distributions, coordinate-space
correlation functions). The qualitative difference between valence and sea
quark intrinsic p_T distributions could be observed experimentally, by
comparing the transverse momentum distributions of selected hadrons in
semi-inclusive deep-inelastic scattering, or those of dileptons produced in pp
and pbar-p scattering.
