  The possibility of measuring the parity-violating asymmetry in M\oller
scattering with sufficient accuracy to determine sin^2 theta_W to 0.1% offers a
complementary path to the discovery of new physics to that followed at high
energy colliders. We present a new calculation of the gamma-Z box contribution
to parity-violating electron-proton scattering, which constitutes an important
uncertainty in computing the background to this process. We show that while the
gamma-Z correction grows rapidly with energy, it can be relatively well
constrained by data from parity-violating inelastic scattering and parton
distribution functions.
