  Number of monitoring observations of continuum emission from Active Galactic
Nuclei (AGNs) have been made in optical--X-ray bands. The results obtained so
far show (i) random up and down on timescales longer than decades, (ii) no
typical timescales of variability on shorter timescales and (iii) decreasing
amplitudes as timescales become shorter. The second feature indicates that any
successful model must produce a wide variety of shot-amplitudes and -durations
over a few orders in their light curves. In this sense, we conclude that the
disk instability model is favored over the starburst model, since fluctuations
on days are hard to produce by the latter model.
  Inter-band correlations and time lags also impose great constraints on
models. Thus, constructing wavelength and time dependent models remains as a
future work.
