  We discuss two important instability mechanisms that may lead to the
limit-cycle oscillations of the luminosity of the accretion disks around
compact objects: ionization instability and radiation-pressure instability.
Ionization instability is well established as a mechanism of X-ray novae
eruptions in black hole binary systems but its applicability to AGN is still
problematic. Radiation pressure theory has still very weak observational
background in any of these sources. In the present paper we attempt to confront
the parameter space of these instabilities with the observational data. At the
basis of this simple survey of sources properties we argue that the radiation
pressure instability is likely to be present in several Galactic sources with
the Eddington ratios above 0.15, and in AGN with the Eddington ratio above
0.025. Our results favor the parameterization of the viscosity through the
geometrical mean of the radiation and gas pressure both in Galactic sources and
AGN. More examples of the quasi-regular outbursts in the timescales of 100
seconds in Galactic sources, and hundreds of years in AGN are needed to
formulate firm conclusions. We also show that the disk sizes in the X-ray novae
are consistent with the ionization instability. This instability may also
considerably influence the lifetime cycle and overall complexity in the
supermassive black hole environment.
