Maser Action in Interstellar OH

Since 1965 we have been unravelling the curious properties of the intense 18 cm emission lines observed from OH molecules. We are still without a quantitative theory of the emission that might tie the threads together and provide guidelines for the observational work. The most plausible hypothesis is that we are dealing with naturally occurring interstellar gas masers. I want to present some of the latest evidence we have in favour of the maser action, and to indicate where it is inadequate. I will concentrate on work done in 1967, as the earlier work has been discussed in detail elsewhere.