  The sensitivity, stability, and uniformity of calibration of the VLBA has
revolutionized parsec-scale polarization studies of AGN jets. Not only does
polarization probe the magnetic field structures of jets, serving as a
hydrodynamic tracer of shocks, bends, and shear, but polarization also probes
the medium through which it propagates by encoding the signature of Faraday
effects along the line of sight. I review advances made by the VLBA in studying
the polarization of jets to probe their magnetic field structures, properties
of the jet plasma, and properties of the external environment. This review
covers both linear and circular polarization and is set in the context of
outstanding questions in the field.
