  I discuss the transfer of polarized synchrotron radiation in relativistic
jets. I argue that the main mechanism responsible for the circular polarization
properties of compact synchrotron sources is likely to be Faraday conversion
and that, contrary to common expectation, a significant rate of Faraday
rotation does not necessarily imply strong depolarization. The long-term
persistence of the sign of circular polarization, observed in some sources, is
most likely due to a small net magnetic flux generated in the central engine,
carried along the jet axis and superimposed on a highly turbulent magnetic
field. I show that the mean levels of circular and linear polarizations depend
on the number of field reversals along the line of sight and that the gradient
in Faraday rotation across turbulent regions can lead to "correlation
depolarization''. The model is potentially applicable to a wide range of
synchrotron sources. In particular, I demonstrate how the model can naturally
explain the excess of circular over linear polarization in the Galactic Center
(Sgr A*) and the low-luminosity AGN M81*.
