Presumably due to a cosmic accident, circumstellar masers are almost exclusively found in oxygen rich stars with [0/C] >1. The exception here is the SiS maser found towards IRC 10216 (Henkel et al. (1985)) but I will ignore this in what follows and concentrate this review upon the circumstellar shells where OH, H2O, and SiO masers are found in profusion. Such objects are thought to be on the asymptotic giant branch (AGB) of the Herzsprung-Russell diagram and show evidence for rapid mass loss. The derived mass loss rates vary from approximately 10−7 solar masses per year for the optically well studied Mira variables in the solar neighbourhood (Distances <1 kpc) to as high as 10−4 solar masses per year for the optically invisible OH-IR stars and for some supergiants. of these objects, the supergiants appear to be more luminous than the rest (105 L rather than 104 L) and their central stars have masses of order 10 M⊙ as compared to most Mira variables whose mass is probably in the range 1–2 M⊙. Most OH-IR stars are likely to be a late stage in the evolution of solar mass type objects where the mass loss rate suddenly increases. As their name indicates, they are characterised by their strong OH-maser (1612 MHz) emission as well as by their radiation at far infrared wavelengths (20–50 μ). They may well be on the way to becoming planetary nebulae (see Pottasch, 1984 for an account of their probable evolution).