Observations of the angular distribution of the water masers in the nucleus of NGC4258 reveal the presence of a thin molecular disk in nearly perfect Keplerian orbit (Miyoshi et al., 1995; Moran et al., 1995). About 300 galaxies have been searched for nuclear water masers to a limiting sensitivity of about 0.1 Jy (e.g., Braatz, 1996); and 16 masers have been detected. The maser imaged by Miyoshi et al. (1995) offers the best example of disk structure. VLA data of NGC1068 shows evidence of disk structure (Gallimore et al., 1996; Greenhill & Gwinn, 1996) and NGC2639 has drifting features, which may be due to centripetal acceleration (Wilson et al., 1995).