Robustness of a controller is very important for industrial process control. It is defined as capability of the system to be unresponsive to component variation. It allows to design feedback system based on strongly simplified models. Controller designed for particular set of parameters is said to be robust if it is capable of working under diverse set of assumptions. It also refers to control of unknown plants with unknown dynamics subject to unknown disturbances. By using robustness analysis one can investigate effect of model uncertainty on the closed loop stability and performance of the control system i.e. it determines whether control system perform satisfactorily over complete operating range or not. This book presents the effect of model perturbations on controller performance in great detail.