**title** Asp Conference Series, Vol. **volume**, **publication Year** **editors** Extragalactic H2o Masers

Study of extragalactic H 2 O masers has progressed significantly in the 25 years since their discovery. Existing in star forming regions and in the accretion disks supermassive black holes, they are familiar and unfamiliar at the same time. A review of how our understanding has grown, up to the present day, is followed by comments on future prospects. Galactic water maser emission in regions of star formation had been known for about a decade (Cheung et al. 1969) when Churchwell et al. (1977) discovered the first extragalactic source, in M 33 (Table 1). Several unsuccessful surveys of nearby galaxies had been conducted before the Churchwell et al. work (Dickinson & Chaisson 1971; Sullivan 1973; Andrew et al. 1975), concentrating on nuclei and fields around H II regions in the stellar disks. Ultimately, the location and origin of the maser in M 33 was not surprising, as it lay in IC 133, a prominent star forming region in the outer stellar disk. The discovery of the first maser source in a galactic nucleus led to speculation that it was a beacon of particularly intense star formation (Table 2), of the type observed toward starbursts (Dos Santos & Lépine 1979; see also Moorwood & Glass 1983). This first " nuclear maser " lay in NGC 4945 (Table 1), which is one of the most luminous nearby far-infrared (FIR) galaxies, and which hosts a massive circumnuclear star-forming ring or disk of gas on 100 kpc scales (Moor-wood et al. 1996). Because star formation rate is correlated with FIR emission, the subsequent discovery of more masers in FIR-bright galaxies was apparently easy to understand. Interferometric observations of the structure of nuclear masers demonstrated that they have more compact structure that was expected for emission tied to distributed star formation in a starburst. Claussen & Lo (1986) used the VLA to demonstrate that the masers in NGC 1068 and NGC 4258 were compact on scales of less than several parsecs. Although nuclear starbursts can also be compact , the observed compactness was in fact the first clue pointing to a truly extraordinary phenomenon. In light of the Antonucci & Miller (1985) model for AGN structure, Claussen & Lo conjectured that the emission was tied to an interaction between nuclear outflow and gas in circumnuclear tori.