Only ten years ago a symposium on the special class of stars we now call Luminous Blue Variables (LBV’s) would not have been possible. At that time we were only just beginning to recognize the similarities among the stars we call S Doradus variables, P Cygni stars, and Hubble-Sandage variables, and their importance in the evolution of the most massive stars.
About a decade ago it became clear that the observed H-R diagram has an upper luminosity limit and that η Car, P Cyg, and S Dor are all near the temperature-dependent boundary for hot stars. With hindsight the luminosity boundary can be seen in an H-R diagram for hot stars published by Hutchings (1976), but the full temperature range is necessary to show that it really exists. When more data became available in 1977—1978, including the evolved and cool supergiants, the upper luminosity limit became obvious; and we proposed that an instability causing rapid and unsteady mass loss (i.e., LBV’s) is the basic reason for this boundary (Humphreys and Davidson 1979). We emphasized the temperature-dependence of the boundary for the most luminous hot stars, the lack of cooler counterparts at similar luminosities, the nearly temperature-independent limit to the luminosities of cool hypergiants, and the critical role that objects like η Car, P Cyg, and S Dor may play in the evolution of the most massive stars.