  Amplitude and period variations have been detected in almost all types of
pulsating stars. Many of these modulations are nearly periodic. In RR Lyrae
stars the phenomenon in known as the Blazhko Effect. Because of the observed
similarities, we propose to extend the definition to other groups. We
illustrate the Blazhko Effect with examples from Cepheids, RR Lyrae stars,
White Dwarfs, sdB stars and delta Scuti variables. For delta Scuti stars the
present results indicate the presence of at least two effects: beating of
independent modes with close frequencies and stellar cycles. For period and
amplitude changes with timescales less than one year, the beating hypothesis
explains the observed modulations very well. This has been shown by the
correctly correlated relationship between amplitude and phase changes as well
as the repetitions of these cycles. However, the observed period variations
with longer timescales cannot be due to simple beating between two close
frequencies. For the star AI CVn (= 4 CVn), we can derive accurate annual
frequency values for a number of radial and nonradial modes from 1974 to 2009.
For prograde and retrograde modes, the frequency variations are of similar
size, but with opposite signs. The radial mode shows no (or little) changes.
Fur- thermore, all frequency variations show a reversal around 1990. These
results are consistent with long-term cycles affecting individual modes
differently with some common systematic behavior. The observed change in the
size of rotational splitting is interpreted as a change in the differential
rotation during the stellar cycle.
