  The Drell-Yan process is a standard tool for probing the partonic structure
of hadrons. Since the process proceeds through a quark-antiquark annihilation,
Drell-Yan scattering possesses a unique ability to selectively probe sea
distributions. This review examines the application of Drell-Yan scattering to
elucidating the flavor asymmetry of the nucleon's sea and nuclear modifications
to the sea quark distributions in unpolarized scattering. Polarized beams and
targets add an exciting new dimension to Drell-Yan scattering. In particular,
the two initial-state hadrons give Drell-Yan sensitivity to chirally-odd
transversity distributions.
