  We give an overview of parts of the theory of Hardy spaces from the viewpoint
of signals and systems theory. There are books on this topic, which dates back
to Bode, Nyquist, and Wiener, and that eventually led to the developement of
$H^{\infty}$ optimal control. Our modest goal here is giving a beginner's
dictionary for mathematicians and engineers who know little of either systems
or $H^2$ spaces.
