The M supergiants are rare objects in the solar neighbourhood and consequently until recently their importance has been largely overlooked. It is now being realized that these stars hold the key to extragalactic distance determinations since they can be detected at large distances while the maximum luminosity they attain seems to be remarkably constant and independent of galaxy type (Sandage and Tammann 1974; Humphreys 1978, 1979a, 1979b; Humphreys and Davidson 1979). They are also important in their own right since they are massive stars in the late stages of their evolution and undoubtedly suffer from mass loss and chemical enrichment due to the mixing of processed materials to their surfaces. In this light they can be considered as precursors of the luminous carbon stars and supernovae.