Control of Unstable Single and Multi-Variable Systems gives an overview on occurrence of unstable systems, identification of transfer function models and simple methods of designing PID controllers for such systems. The controller design methods include equating coefficient method, synthesis method, IMC method, pole placement method, stability analysis method, model reference method and two degree of freedom controller method. Several examples are given to illustrate these methods. Methods for the calculation of set point weighting parameter to improve the servo response of the controller and identification of unstable transfer function model by closed loop reaction curve method and optimization method are discussed in detail, with the help of case studies alongside the methods of designing decentralised PID controllers, centralised PID controllers and the methods to evaluate performance, stability and robustness of the controller.