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Naturally occurring water vapor maser emission at 1.35 cm wavelength providesan accurate probe for the study of accretion disks around highly compactobjects, thought to be black holes, in the centers of active galaxies. Becauseof the exceptionally fine angular resolution, 200 microarcseconds, obtainablewith very long baseline interferometry, accompanied by high spectralresolution, < 0.1 km/s, the dynamics and structures of these disks can beprobed with exceptional clarity. The data on the galaxy NGC4258 are discussedhere in detail. The mass of the black hole binding the accretion disk is 3.9times 10^7 solar masses. Although the accretion disk has a rotational period ofabout 800 years, the physical motions of the masers have been directly measuredwith VLBI over a period of a few years. These measurements also allow thedistance from the earth to the black hole to be estimated to an accuracy of 4percent. The status of the search for other maser/black hole candidates is alsodiscussed.Comment: 24 pages, 11 figures, latex, uses aaspp4 style file. To be published  in the Journal of Astronomy and Astrophysics (India), proceedings of the  Discussion Meeting on the Physics of Black Holes, Bangalore, India: December  199

Observational evidence for massive black holes in the centers of active galaxies