As recently as 20 years ago, whether or not active galactic nuclei (AGNs) were powered by accretion onto supermassive black holes was widely regarded as an open question. The theoretical arguments supporting gravitational accretion as the primary source of power in AGNs were in place within a few years of the discovery of quasars. Now, however, it is now generally accepted that supermassive black holes reside at the centers of most, if not all, massive galaxies. The difference between active and quiescent galaxies is due to differences in accretion rate relative to the Eddington rate and thus radiative efficiency. In this talk, I will discuss methods for measuring the masses of supermassive black with emphasis on reverberation mapping which is most generally applicable to accreting supermassive black holes and, in particular, to distant quasars where time resolution can be used as a surrogate for angular resolution.