  Galactic nuclei are well known sources of OH and H2O maser emission. It
appears that intense star formation in ultra-luminous infrared galaxies drives
most OH sources. In contrast, nuclear activity appears to drive most H2O
sources. When H2O emission originates in accretion disk structures, constrained
geometry and dynamics enable robust interpretation of spectroscopic and imaging
data. The principal science includes study of AGN geometry at parsec and
sub-parsec radii and measurement of geometric distances in the Hubble Flow. New
high accuracy estimates of the Hubble constant, "Ho," obtained from maser
distances may enable new substantively improved constraints on fundamental
cosmological parameters (e.g., dark energy).
