  I review the temporal/spectral data of accreting black hole sources paying
most attention to the properties of the temporal variability such as photon
energy dependent auto- and cross-correlation functions, average shot profiles
and hardness ratios, and the Fourier frequency dependent time/phase lags. These
statistics characterize spectral changes at short time scales that are
otherwise impossible to study by direct spectral analysis. The data provide
strong constraints on the theoretical models for X-ray production in accreting
black holes. Models for the spectral variability of the Comptonized component
are briefly reviewed. It is also shown that Compton reflection can have
significant impact on the observed temporal characteristics.
