Since interstellar masers were first discovered in 1965, various attempts have been made to account for the variety of behaviour observed. In particular a number of theoretical models have been advanced to account for the pumping mechanisms but none have gained a wide acceptance.

Over forty molecules have now been identified in the interstellar medium, of these five exhibit maser action; they are detected by virtue of their small angular diameter and very large brightness temperatures > 1013°K. The molecules in question are H20, OH, SiO, CH3OH and CH, of these the Hydroxyl maser presents the widest range of behaviour and therefore if the physical mechanisms involved could be understood these masers could be used to probe the interstellar environment and obtain more information about this important region where new stars are formed