  We present model light curves, Power Spectral Densities (PSD) and time lags
of accreting Black Hole Candidates (BHC) based on a recent model of these
sources. According to our model the observed variability is due, to a large
extent, to the stochastic nature of Comptonization, the basic process
responsible also for the emission of high energy radiation. Our additional
assumption is that the Comptonization process takes place in an extended but
non-uniform atmosphere around the compact object. Our model reproduces the
observed light curves well, in that it provides a good fit to the PSDs and the
overall light curve morphology, indicating, in accordance with observation,
that most of the power resides at time scales $\gsim$ a few seconds while at
the same time one can distinguish shots of a few msec in duration. We suggest
that refinement of this type of model along with spectral and phase lag
information can be used to probe the structure of this class of high energy
sources.
