Circular polarization is produced either in the emission process through the Zeeman effect, or through the transfer of polarized radiation in a magneto-active medium. While the former mechanism provides a direct access to the strength of the magnetic field along the sightline, the conversion of linear to circular polarization reveals particular magnetic field structures or magnetospheric events. In the first part I will present selected science cases. While Stokes V may be strong in OH masers forming in supernova remnants, the Zeeman effect becomes weaker in masers associated with star-forming environments or circumstellar envelopes. As masers probe restricted physical conditions, the Zeeman effect in thermal lines is a valuable analysis tool, but even more difficult to measure. In particular, the instrumental leakage from linear to circular polarization frequently exceeds the intrinsic Stokes V. This statement also holds for the Faraday conversion in AGN. A correction for instrumental polarization across the field of view is thus essential, but hindered by the lack of suitable calibrators. In the second part I will propose calibration schemes for both the image and UV plane.< Leer menos