  Scattering polarized electrons provides an important probe of the weak
interactions. Precisely measuring the parity-violating left-right cross section
asymmetry is the goal of a number of experiments recently completed or in
progress. The experiments are challenging, since A_{LR} is small, typically
between 10^(-4) and 10^(-8). By carefully choosing appropriate targets and
kinematics, various pieces of the weak Lagrangian can be isolated, providing a
search for physics beyond the Standard Model. For other choices, unique
features of the strong interaction are studied, including the radius of the
neutron density in heavy nuclei, charge symmetry violation, and higher twist
terms. This article reviews the theory behind the experiments, as well as the
general techniques used in the experimental program.
