The calibration of the extragalactic distance scale: methods and problems

Measuring distances in the expanding universe by the classical method of `standard candles' creates a calibration problem: what is the power of the standard candles, and how standard are they? Hubble's law has recession velocity proportional to distance from the observer, v = H0d; but what is the value of H0, the Hubble constant? Ten years after the launch of the Hubble Space Telescope (HST), which has made a major impact on this problem, we review progress. The HST Key Project on the extragalactic distance scale has provided a number of calibration points, galaxies at characteristic distances of 10 Mpc. HST has resolved the primary standard candles, Cepheid variable stars, beyond the reach of ground based telescopes. These galaxy `survey markers' in turn calibrate a number of more penetrating distance indicators which reach out an order of magnitude further, sufficiently far that noise in the Hubble flow deltav<