An overview of some recent results in the controller de sign of multivariable linear time-invariant systems, in which uncertainty/perturbation in plant behaviour occurs is made. In particular, the servomechanism problem is considered for various cases varying from the extreme case when the plant is known and fixed, to the opposite extreme when the plant is completely unknown and is allowed to be perturbed. In addition, the chapter includes the case of time lag systems, decentralized systems and the control of systems subject to sensor/actuator failure. The emphasis throughout is to characterize structural results for the servomechanism problem, e.g. to determine the existence of a solution to the problem and to determine the controller structure required to solve the problem under different conditions. The chapter concludes with a proposed design method for obtaining realistic robust controller design.