In the literature there is now a wealth of images of AGN in both linear and circular polarization at milliarcsecond resolution of many sources at multiple epochs and wavelengths. This review is a broad overview of our current state of knowledge of the polarization properties of AGN jets, and of what we can infer about the structure of their magnetic fields and the distribution of Faraday rotating material, and also what is premature to infer. We suggest that while many of the observations of transverse rotation measure gradients may be correct, they have in general been observed with neither enough resolution nor sensitivity to be incontrovertible. Fortunately, this could be remedied using the new broad-band receivers on the VLBA. We also argue that while a transverse rotation measure gradient is a strong indicator of a toroidal component of magnetic field, it does not necessarily imply a helical magnetic field. In jets where the dominant field component appears to be parallel to the jet, it more likely consists of sheared loops or compressed random fields that are not vector ordered. Finally, we draw attention to the power of imaging in all four Stokes parameters at multiple frequencies to constrain the three-dimensional magnetic field structure and the particle content of jets and measure fundamental physical properties. The linear polarization of the radio emission from AGN jets (assumed to be synchrotron radiation) tells us about the structure and order of the magnetic field. Especially on parsec scales, these are intimately related to the launching, acceleration and collimation of the jets (see the review by D. Meier, these proceedings). Monitoring reveals the evo- lution of the magnetic field, and the formation and dissi- pation of shocks. Faraday rotation measurements are an important diagnostic of thermal electrons (and lower en- ergy relativistic electrons) inside and outside the jet, and have revealed a torroidal component of the magnetic field in some sources. Observations of circular polarization can tell us about the positron fraction in the relativistic electron population and the vector-ordering of the magnetic field. The latter then permits an estimate of the net magnetic flux in the jet, and hence (because it is a conserved quantity) the magnetic flux at the central black hole-accretion disk system. It should be noted that none of the physical pa- rameters listed in this paragraph can be gleaned from total intensity images. With the advent of the VLBA in 1993, polarimetry at milliarcsecond resolution came of age. Earlier polariza- tion results, and some of the first polarization results from the VLBA, were discussed in the 1997 Annual Reviews article by Zensus (1). A conference celebrating the 10th birthday of the VLBA (2) featured 18 papers devoted to the polarization properties of AGN on parsec scales. These in- cluded observations of rotation measures, transverse gra-