UC Irvine

In the first fifty years after Edwin Hubble announced a linear relationship between distances and redshifts of external galaxies, the accepted value of his constant dropped by (or the Universe expanded and aged by) a factor of 5 to 10. More recently, different groups, often using nearly the same data, have passionately defended distance scales that differ by about a factor of two. The sections of this review explore (1) the history of extragalactic distance scales, (2) the relationships between the Hubble constant, H0, and other cosmological parameters, (3) types of distance indicators, (4) ways of measuring distances in practice, (5) values of H0 reported recently on the basis of these methods, (6) the continuing discrepancies between the 'long' and 'short' distance scales, and (7) prospects for future convergence on a single, global value of H, so that we can all get back to doing other things. The units of the Hubble constant are km s-1 Mpc-1 (or reciprocal time), and no one now strongly favors any value outside the range 40-90 km s-1 Mpc-1 (time scales of 11-25 Gyr).