Abstract: This paper systematically analyses the negative influence of delays in NCSs (networked control systems) on system stability and other performances, and gives some methods to compensate for these delays. The systems are classified according to the characteristics of their delays-shorter or longer than a sample period, constant or variable, random or determinate, and the independent or dependent features under some conditions are discussed. The stochastic control methods to guarantee the mean square stability of the system are summarized. It is pointed out that the model method to study NCS is various, an d that the coherency property of all these methods is worthy of discussing.