  We review the class of galactic supernova remnants which show strong
interactions with molecular clouds, revealed through shock-excited hydroxyl
masers. These remnants are preferentially found among the known GeV and TeV
detections of supernova remnants. It has been argued that the masers trace out
the sites of hadronic particle acceleration. We discuss what is known about the
physical conditions of these shocked regions and we introduce a potential new
maser tracer for identifying the sites of cosmic ray acceleration. This review
includes a reasonably complete bibliography for researchers new to the topic of
shock-excited masers and supernova remnants.
