In recent years, there is a hot problem on time-varying of the plant which becomes more and more increasingly
concerned by control areas. Thus, a series of new control methods are proposed continuously. Firstly, the classification
methods of the time-varying plants are analyzed and several key issues on such plants are summarized. Then two different
methods on model description are proposed. Based on these, according to the differences on the time-varying characteristics,
different control approaches are analyzed and the status and performance characteristics of time-varying system are surveyed.
Finally, the perspective of time-varying control theory and associated research topics are briefly explored and discussed.