  We present a simple feedback description of parametric instabilities which
can be applied to a variety of optical systems. Parametric instabilities are of
particular interest to the field of gravitational-wave interferometry where
high mechanical quality factors and a large amount of stored optical power have
the potential for instability. In our use of Advanced LIGO as an example
application, we find that parametric instabilities, if left unaddressed,
present a potential threat to the stability of high-power operation.
