The 22 GHz radio emission from luminous extragalactic water masers originates in dense and warm gas clouds within a few parsecs from the nuclear engines of their parent galaxies. These masers trace circumnuclear accretion disks (e.g. NGC4258, UGC3789), the inner parts of relativistic jets (e.g. Mrk348), or nuclear outflows (Circinus) that are associated with AGN. In contrast to optical and ultraviolet radiation, the radio photons can penetrate the enormous column densities of gas and dust that often obscure the line of sight to the nucleus. This, together with the high brightness temperature and small size of the maser hotspots, makes H2O maser emission a suitable tool to investigate the geometry, kinematics, and excitation conditions of the gas in the immediate vicinity of supermassive black holes. To date, most such studies have targeted radio quiet AGN (Seyfert 2 or LINER) in the local Universe (z<0.06). However, our discovery of a water maser in the type 1 quasar MGJ0414+0534 at z=2.64 demonstrated that H2O masers can also be detected at higher redshifts. In this talk, I will review the main characteristics of water masers in AGN and the most important results obtained so far in this field of research. Furthermore, I will present my ongoing studies, especially those related to a long term monitoring of the high redshift water maser in MGJ0414+0534 and to the investigation of the correlations between X-ray and water maser emission in AGN.