  Feedback is a most important concept in control systems, its main purpose is
to deal with internal and/or external uncertainties in dynamical systems, by
using the on-line observed information. Thus, a fundamental problem in control
theory is to understand the maximum capability and potential limits of the
feedback mechanism. This paper gives a survey of some basic ideas and results
developed recently in this direction, for several typical classes of uncertain
dynamical systems including parametric and nonparametric nonlinear systems,
sampled-data systems and time-varying stochastic systems.
