  Theoretical research on extragalactic masers (and hence this review) is
almost entirely directed toward 22 GHz water masers in circumnuclear disks with
the focus being the near ideal masing disk at the nucleus of the galaxy
NGC4258. The discussion here is organized around (1) the excitation and
conditions for the masers, (2) the spatial and spectral appearance of the maser
emission, and (3) the mass accretion rate and structure of the disk. In
addition, a summary is given of the basic physics (spectral linebeadths and
maser polarization) that underlies certain interpretations of these and other
astrophysical masers.
