Does the thermal disk instability operate in active galactic nuclei?

We examine all possible stationary, optically thick, geometrically thin acc retion disk models relevant for active galactic nuclei (AGN) and identify t he physical regimes in which they are stable against the thermal-viscous hy drogen ionization instability. Self-gravity and irradiation effects are inc luded. We find that most if not all AGN disks are unstable. Observed AGN th erefore represent the outburst state, although some or all quasars could co nstitute a steady population having markedly higher fueling rates than othe r AGN. This finding has important implications for the AGN mass supply and for the presence of supermassive black holes in nearby spirals.