  Basic aspects of the relationship between the magnetic field and polarized
maser radiation are described with the emphasis on interpreting the observed
spectra. Special attention is given to three issues--the limitations on the
applicability of the classic solutions of Goldreich, Keeley & Kwan (1973),
inferring the strength of the magnetic field from the circular polarization
when the Zeeman splitting is much less than the spectral linebreadth
(especially for SiO masers), and the significance of the absence of components
of the Zeeman triplet in the spectra of OH masers in regions of star formation.
