As a hot research topic, a continuous-time control system hasbeen gained much attention due to the fact that it has been widely and successfullyused in a great number of industrial applications. Time-delays are ubiquitous in industrial control systems. While the effectiveness of a control system closely relies on timely delivery of control signals, small time-delays are inherent in industrial sensors and actuators. Furthermore, in somelarge scale (distributed) control systems, such as manufacturing plants and power generation plants, communication networks are employed to exchange information and control signals between spatially distributed system components, As a result, substantial time-delay isinduced inevitably. Traditionally time-delays have been thought to have a deteriorated effect on both the stability and the performance of controlled systems, and much research has been conductedin attempting to eliminate them, compensate for them, or nullify their presence.

In recent years, a new research area has emerged that makes use of time-delay for practical engineering controlpurposes, such as stabilisation of oscillatory systems, controlling chemical reactors, active control of offshore structures subject toself-excited nonlinear hydrodynamic force, active control of buildings against earthquakes, and controlling aeroelastic systems in aerospace engineering. It has beenshown that introducing time-delay in control is possibly beneficial to achievingsuperior control performance, e.g. regulating chaos into their unstable periodic orbits, vibration control of resonators, active control of offshore structures/buildings, and aeroelastic systemcontrol.