For nearly a century astronomers have expended considerable effort to determine the size of the Milky Way. This effort is worthwhile because any change in the value of the distance from the Sun to the center of the Galaxy, Ro, has widespread impact on astronomy and astrophysics. All distances determined from observed radial velocities and a rotation model of the Galaxy are directly proportional to Ro. Most estimates of the gravitational and luminous mass of the Galaxy scale with Ro. Similarly, the mass and luminosity of objects within the Galaxy, such as giant molecular clouds and the nonthermal source at the Galactic center depend on Ro• On a larger scale, since extragalactic distances are based on Galactic calibrations, the Hubble constant and Ro are interrelated. Indeed, it may be possible to use the size of the Milky Way as an extragalactic "meter stick" and determine distances to similar spiral galaxies (e.g. de Vaucouleurs 1 983a,b; van der Kruit 1 986). Historically, astronomers have measured distances to nearby stars, used these distances to calibrate their luminosities, and estimated Ro from the spatial distributions of stars and globular clusters. Recently, however, direct measurements of Ro have become feasible and the possibility of using Ro as a distance standard has emerged. For example, if one knew Ro very accurately, one could recalibrate the absolute magnitudes of Cepheid,