In this paper, the authors summarize recent work in which they demonstrate that the Compton subprocess, q + g 2![gamma]* + q also dominates the Drell-Yan cross section in polarized and unpolarized proton-proton reactions for values of the transverse momentum Q{sub T} of the pair that are larger than roughly half of the pair mass Q, Q{sub T}> Q/2. The Drell-Yan process is therefore a valuable, heretofore overlooked, independent source of constraints on the spin-averaged and spin-dependent gluon densities. Although the Drell-Yan cross section is smaller than the prompt photon cross section, massive lepton pair production is cleaner theoretically since long-range fragmentation contributions are absent as are the experimental and theoretical complications associated with isolation of the real photon. Moreover, the dynamics of spin-dependence in hard-scattering processes is a sufficiently complex topic, and its understanding at an early stage in its development, that several defensible approaches for extracting polarized parton densities deserve to be pursued with the expectation that consistent results must emerge.