This paper was presented at a colloquium entitled "Physical Cosmology," organized by a committee chaired by David

Five methods of estimating distances have demonstrated internal reproducibility at the level of 5-20% rms accuracy. The best of these are the cepheid (and RR Lyrae), planetary nebulae, and surface-brightness fluctuation tech- niques. Luminosity-line width and D.-r methods are less accurate for an individual case but can be applied to large numbers of galaxies. The agreement is excellent between these five procedures. It is determined that Hubble constant Ho = 90 ? 10 km-s- 1 Mpc- 1 (1 parsec (pc) = 3.09 x 1016 m). It is difficult to reconcile this value with the preferred world model even in the low-density case. The standard model with Q = 1 may be excluded unless there is something totally misunderstood about the foundation of the distance scale or the ages of stars. The Hubble constant, Ho, is a measure of the current scale of the universe compared with the expansion rate: the charac- teristic recessional velocity between two objects equals the Hubble constant times the distance separating the objects. The inverse of this constant is taken to be on order of the age of the universe because it gives the look-back time to when all matter in expansion can be traced back to the same point, assuming no modifying forces. There is a common perception that the value of the Hubble constant is only poorly known, confined to the range 50 < Ho < 100 km s-l-Mpc-1 (1 parsec (pc) = 3.09 x 1016 m). It will be argued here that this perception is incorrect-that there is now a concordance of good methods with low internal scatter and excellent external consistency that lead to Ho = 90 + 10 km-s- lMpc-1. The inverse characteristic age for the uni- verse is Ho 1 = 11 ? 1 gigayears (Gyr). This value is different from the generally accepted age of