Hubble's celebrated constant, which relates the recessional velocities of galaxies to their distances, is perhaps the most important number in extragalactic astronomy. Using it, along with their favorite models, astronomers and cosmologists derive the age of the universe, estimate its size, calculate the luminosity of quasars and much more. Unfortunately, astronomers are far from agreeing on a value for Hubble's constant. Some believe that H0 is about 50 km/sec Mpc; others think it is closer to 100 km/sec Mpc. Depending on the details of the cosmological model chosen, this can lead to a discrepancy of a factor of 2—ten billion years or so—in the age of the universe.