Recent spectroscopic and VLBI-imaging observations of bright extragalactic water maser sources have revealed that the megamaser emission often originates in thin circumnuclear disks near the centers of active galactic nuclei (AGNs). The imaged maser systems serve as a unique diagnostic of the mass distribution in these galactic nuclei and have provided some of the strongest evidence to date for the presence of massive black holes in AGNs. This talk will focus on the implications of the maser observations to the nature of circumnuclear disks in active galaxies. Among the topics that will be addressed are the excitation mechanism of the masers; their geometry, kinematics, and physical properties; and their potential for testing mass-accretion models of AGNs. Applications to specific objects and the relationship between extragalactic and galactic water-maser sources will also be discussed.