  We discuss the production of circular polarization in compact radio sources
both by the intrinsic mechanism and by Faraday conversion. We pay particular
attention to the magnetic field structure, considering partially ordered fiel
ds and Laing sheets, and distinguishing between uniform and unidirectional
fields. (The latter can be constrained b y flux conservation arguments.) In
most cases, Faraday conversion is the more important mechanism. Conversion
opera tes on Stokes U, which can be generated by internal Faraday rotation, or
by magnetic field fluctuations, which can therefore produce circular
polarization even in a pure pair plasma. We also show that the spectrum of
circular pola rization in an inhomogeneous jet can be quite different from that
in a uniform source, being flat or even inverted.
