Maser polarization observations have been successfully used to characterize magnetic fields towards a variety of astrophysical objects. Circular polarization yields the magnetic field strength of the maser source, and linear polarization yields information on the magnetic field morphology. Linear polarization can be produced when the maser saturates or through its anisotropic pumping. We present a comprehensive model of the polarization of masers. In contrast to regular excitation modeling, we relax the assumption of isotropically populated level populations, and model both the total population and level alignments. Through this approach, we obtain quantitative estimates on the anisotropic pumping of a variety of maser sources. In this way, the maser polarization may be related to the gas density, temperature, geometry and the magnetic field. Using the results of our modeling, we discuss, and give predictions, of the polarization of SiO, methanol, and water (mega)masers.
