The system under consideration is shown in block diagram form in Fig. 1.1. P denotes a given system, usually called the plant, whose output is y and whose inputs are the controller output u and disturbance d (for example P is the model of an airplane whose output is its flight path, y, due to its control surface variations, u, and wind disturbances d). Fr is an external command. G is the controller whose tasks, as will be shown in the next section are (i) reduction of the plant output response, y, to disturbance d; and (ii) reduction of the response variation, y, from the desired output, if the plant deviates from its assumed model P. H models the sensor which measures the plant output y, and in general is a good approximation to the plant output, y. Noise n, which is not correlated to y, is added to the measurement Hy.