  Luminous water maser emission in the 6_(16)-5_(23) line at 22 GHz has been
detected from two dozen galaxies. In all cases the emission is confined to the
nucleus and has been found only in AGN, in particular, in Type 2 Seyferts and
LINERs. I argue that most of the observed megamaser sources are powered by
X-ray irradiation of dense gas by the central engine. After briefly reviewing
the physics of these X-Ray Dissociation Regions, I discuss in detail the
observations of the maser disk in NGC 4258, its implications, and compare
alternative models for the maser emission. I then discuss the observations of
the other sources that have been imaged with VLBI to date, and how they do or
do not fit into the framework of a thin, rotating disk, as in NGC 4258.
Finally, I briefly discuss future prospects, especially the possibility of
detecting other water maser transitions.
