Combined polarization imaging of radio
jets from Active Galactic Nuclei (AGN) in circular and linear
polarization, also known as full Stokes imaging, has the potential to
constrain both the magnetic field structure and particle properties of
jets. Although only a small fraction of the emission when detected,
typically less than a few tenths of a percent but up to as much as a
couple of percent in the strongest resolved sources, circular
polarization directly probes the magnetic field and particles within the
jet itself and is not expected to be modified by external screens. A
key to using full Stokes observations to constrain jet properties is
obtaining a better understanding of the emission of circular
polarization, including its variability and spectrum. We discuss what we
have learned so far from parsec scale monitoring observations in the
MOJAVE program and from multi-frequency observations of selected AGN.