The history of OH megamaser (OHM) research is reviewed, and recent developments in the field are summarized. Particular attention is paid to results from VLBI, and the recognition of a wide range of maser properties within a single galaxy. A peculiar characteristic of compact parsec-scale OHM features is a very broad velocity width, which poses a challenge for radiative pumping models, and suggests filamentary geometries for the emitting clouds. The compact masers may be associated with shocks in a violent nuclear environment, and collisional pumping may play a role. A link between the compact masers and newly formed active galactic nuclei (AGNs) cannot be ruled out. The low-gain amplification model of the diffuse maser component is strongly supported by recent data, and detailed information about the parameters of the amplifying screen is starting to emerge. OHM can be used as powerful probes of dusty, obscured IR galaxy nuclei, and the prospects for these objects being detectable, and therefore useful as probes, at cosmological distances is discussed.