  Flickering is a universal phenomenon in accreting astronomical systems which
still defies detailed physical understanding. It is particularly evident in
cataclysmic variables (CVs). Attempting to define boundary conditions for
models, the strength of the flickering is measured in several thousand light
curves of more than 100 CVs. The flickering amplitude is parameterized by the
FWHM of a Gaussian fit to the magnitude distribution of data points in a light
curve. This quantity requires several corrections before a comparison between
different sources can be made. While no correlations of the flickering strength
with simple parameters such as component masses, orbital inclination or period
were detected, a dependence on the absolute magnitude of the primary component
and on the CV subtype is found. In particular, flickering in VY Scl tpye
novalike variables is systematically stronger than in UX UMa type novalikes.
The broadband spectrum of the flickering light source can be fit by simple
models but shows excess in the $U$ band. When the data permitted to investigate
the flickering strength as a function of orbital phase in eclipsing CVs, such a
dependence was found, but it is different for different systems. Surprisingly,
there are also indications for variations of the flickering strength with the
superhump phase in novalike variables with permanent superhumps. In dwarf
novae, the flickering amplitude is high during quiescence, drops quickly at an
intermediate magnitude when the system enters into (or returns from) an
outburst and, on average, remains constant above a given brightness threshold.
