  Luminous extragalactic masers are traditionally referred to as the
`megamasers'. Those produced by water molecules are associated with accretion
disks, radio jets, or outflows in the nuclear regions of active galactic nuclei
(AGN). The majority of OH maser sources are instead driven by intense star
formation in ultra-luminous infrared galaxies, although in a few cases the OH
maser emission traces rotating (toroidal or disk) structures around the nuclear
engines of AGN. Thus, detailed maser studies provide a fundamental contribution
to our knowledge of the main nuclear components of AGN, constitute unique tools
to measure geometric distances of host galaxies, and have a great impact on
probing the, so far, paradigmatic Unified Model of AGN.
