  The 25 years following the serendipitous discovery of megamasers have seen
tremendous progress in the study of luminous extragalactic H$_2$O emission.
Single-dish monitoring and high resolution interferometry have been used to
identify sites of massive star formation, to study the interaction of nuclear
jets with dense molecular gas and to investigate the circumnuclear environment
of active galactic nuclei (AGN). Accretion disks with radii of 0.1--3 pc were
mapped and masses of nuclear engines of order 10$^{6}$--10$^{8}$ M$_{\odot}$
were determined. So far, $\sim$50 extragalactic H$_2$O maser sources have been
detected, but few have been studied in detail.
