Of course, while distances to individual galaxies are of value in their own right, there is a more compelling reason for undertaking this extremely time-consuming task of Cepheid discovery and calibration. That goal is the Hubble constant, a measure of the size-scale and indeed the time-scale of the Universe. This goal was deemed sufficiently important that the design of the Hubble Space Telescope was in large measure constrained to meet the minimum requirements needed to undertake this task from space: to discover Cepheids in galaxies well beyond the Local Group, out to and including the Virgo and Fornax clusters.
Even so, one or more secondary distance indicators are needed to get to ``cosmologically significant'' distances where the pure Hubble flow can be reliably measured. And it is generally accepted that these secondary distance indicators are best calibrated by the Cepheid PL relation. At the moment secondary methods with small measured internal dispersions are the best contenders for confidently extending the distance scale, with the intent of determining the value of the Hubble constant and placing a credible uncertainty. They are: the Tully-Fisher relation, Type II SN, Type Ia SN, the surface brightness fluctuation technique and the Faber-Jackson relation. The planetary nebula luminosity function, although showing great promise as a precise distance indicator, has not been pressed to distances any further than those already directly probed by Cepheids themselves. The latter three, because they apply primarily to early-type spirals, S0 galaxies or ellipticals might seem to be outside of the sphere of influence offered by the Cepheid calibration, but this is not strictly true. With the availability of HST a volume of space at least one thousand times larger than is regularly available to ground-based telescopes is now accessible. This volume includes several groups of galaxies that are known to contain both early-type systems and late-type spirals. Confirmed group membership then allows Cepheid distances to be brought into the calibration of galaxies that do not themselves contain Cepheids. This has in fact been the case in several attempts to calibrate these same relations from the ground. Specifically, M31 and M81 (which have Cepheid-based distances) with their significant bulge luminosity have been used to calibrate the PNLF (Ciardullo et al. 1989) and an application of the Faber-Jackson relation (Dressler 1987). For the surface-brightness-fluctuation technique the apparent association of M32 again with M31 has been used as a calibrating path. For the future, obvious groups with a mix of Hubble types, worthy of detailed distance determinations are the Centaurus (NGC 5128) Group, and the Leo I (M96 = NGC 3368) Group; with the NGC 2841 Group, the NGC 1023 Group and the NGC 2997 Group offering some interesting potential as well.