  After almost five decades of study of parity violation in hadronic systems,
the determination of the seven weak meson-nucleon couplings is still
incomplete. Whereas parity violation in nuclear systems is complicated by the
intricacies of QCD, measurements of parity violation in the much simpler
proton-proton system are more straightforward to interpret. We now have three
such precision pp experiments at 13.6, at 45, and 221 MeV. Today there are also
better possibilities for theoretical interpretation using effective field
theory. In electron-proton scattering, parity violating ep experiments such as
SAMPLE, G0, HAPPEX, and PVA4 have already shown that the strange quark
contributions to the charge and magnetization distributions of the nucleon are
tiny. When analyzed together, the results have also greatly improved knowledge
of the proton's "weak charge" (Q^p_weak = 1-4sin^2\theta_W at tree level). The
Q^p_weak experiment at JLab will further improve this, determining the proton's
weak charge to a precision of about 4%. Such a precision will either establish
conformity with the Standard Model of quarks and leptons or point to New
Physics. Following the upgrade of CEBAF at JLab to 12 GeV, a parity violating
electron-electron (Moller) scattering experiment similar to SLAC E158, will
measure the weak charge of the electron and hence sin^2\theta_W at low energy
with a precision comparable to the most precise individual measurements at the
Z0 pole (to about +/- 0.00025). This experiment will be complementary to
Q^p_weak in terms of sensitivity to New Physics.
