Supernovae have a profound and perhaps dominating effect on the morphology, kinematics, and metallicity of galaxies. The impact of supernova shocks on surrounding molecular clouds is also thought to trigger new generations of star formation. A critical ingredient in such interactions and, indeed, all aspects of supernova remnant (SNR) evolution are magnetic fields. In recent years, OH (1720 MHz) masers have been used as signposts for the interaction of SNRs with molecular gas. In addition to tracing SNR/molecular cloud interactions, the OH (1720 MHz) maser line also provides a unique opportunity to measure the strength of the post-shock magnetic field via Zeeman splitting. Recent results from efforts to both detect the magnetic fields and resolve the maser spot sizes of OH (1720 MHz) masers have yielded magnetic field detections between 100 and 4000 uG toward 10 different SNRs and large spot sizes of about 10^15cm. These observational results, which represent the first direct magnetic field estimates for the post-shock gas in SNRs, as well as the resulting implications for maser theory, are reviewed.