  We show that the initial-state interactions from gluon exchange between the
incoming quark and the target spectator system lead to leading-twist
single-spin asymmetries in the Drell-Yan process. The QCD initial-state
interactions produce a $T-$odd spin-correlation between the target spin and the
virtual photon production plane which is not power-law suppressed in the
Drell-Yan scaling limit. The origin of the single-spin asymmetry in $\pi
p^\uparrow \to \ell^+ \ell^- X$ is a phase difference between two amplitudes
coupling the proton target with $J^z_p = \pm {1\over 2}$ to the same
final-state, the same amplitudes which are necessary to produce a nonzero
proton anomalous magnetic moment. The calculation requires the overlap of
target light-front wavefunctions differing by one unit of orbital angular
momentum projection $L_z;$ thus the SSA in the Drell-Yan reaction provides a
direct measure of orbital angular momentum in the QCD bound state. The
single-spin asymmetry predicted for the Drell-Yan process $\pi p^\uparrow \to
\ell^+ \ell^- X$ is similar to the single-spin asymmetries in deep inelastic
semi-inclusive leptoproduction $\ell p^\uparrow \to \ell' \pi X$ which arises
from the final-state rescattering of the outgoing quark.
