In the past three years, observations of extragalactic water masers have demonstrated the extraordinary power of spectral line radio interferometry to probe the inner regions of active galaxies on subparsec scales. In the case of the active galaxy NGC 4258, the observations of Miyoshi et al. (1995) have established the presence of a thin, slightly warped, and Keplerian circumnuclear accretion disk that orbits a central object of mass 3.6 × 107M⊙. In addition to their immense value as kinematic

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... robes, extragalactic water masers also provide valuable constraints upon the physical and chemical conditions in the masing circumnuclear gas, yielding an estimate of the mass accretion rate through the circumnuclear disk of NGC 4258. These recent observational and theoretical developments are reviewed here.