The study of Cepheids in external galaxies has been a continuous probing of the near-by distance scale accompanied by a refining and redefining of the Period-Luminosity relation itself. For example, the nearest two late-type galaxies of the Local Group, the Magellanic Clouds have always been the testbed for calibrations, from the pioneering optical studies of Leavitt (1907) to the recent infrared calibration of McGonegal et al. (1982) three quarters of a century later.
With time the data have improved in quality and in quantity but the methods of analysis and interpretation have so proliferated that the field might appear to be less well defined than it was at the turn of the century. Discussions of the problems have been so open that many have been driven to the conclusion that it is perhaps better to gauge the distances to galaxies using a variety of indicators, of secondary quality, rather than risk systematic errors at the start, through a complete reliance on one primary distance indicator. That situation, I believe, now has the potential for reversing itself.