After discussing various principles of suspensions with variable dampers and springs as well as active components, mathematical models of these systems are derived. It is shown how the unknown parameters can be obtained experimentally through parameter estimation by using body accelerometers, wheel accelerometers, and suspension deflection sensors in different combinations. Experimental results are described for suspensions on a test rig and in cars driving over road surfaces. Through recursive parameter estimation these parameters can be obtained on-line in real time. Then, feedback principles are derived for controlling the damping ratio of dampers with proportional magnetic valve actuators. The mathematical models are then used for fault detection and diagnosis of the damper by combining parameter estimation and parity equation methods.