  The standard accretion discs are known to be thermally and viscously unstable
over a certain range of temperatures. In the inner disc regions there may
develop radiation pressure driven instability, which is possibly related to the
rapid variability detected in AGNs in the UV range. In the outer disc develops
the ionization instability, similar to that in the cataclysmic variables, but
operating on much longer timescales. Due to this process the spectrum of the
accretion disc differs from that of a stationary one.
