  The control of individual quantum systems is now a reality in a variety of
physical settings. Feedback control is an important class of control methods
because of its ability to reduce the effects of noise. In this review we give
an introductory overview of the various ways in which feedback may be
implemented in quantum systems, the theoretical methods that are currently used
to treat it, the experiments in which it has been demonstrated to-date, and its
applications. In the last few years there has been rapid experimental progress
in the ability to realize quantum measurement and control of mesoscopic
systems. We expect that the next few years will see further rapid advances in
the precision and sophistication of feedback control protocols realized in the
laboratory.
