Simplifying assumptions regarding the nature of certain types of continuous processes lead to transfer functions which are amenable to analysis. Criteria for optimum control based on the transient responses of such systems when under proportional plus derivative plus integral control are given.The effects of integral control at low frequencies are studied, and it is shown how conditional stability can be avoided when the process contains an integration. A low-frequency damped oscillation which can occur with three-term control is investigated, and the condition for its elimination is derived.An analytical method is developed which, for any given integral control setting, makes it possible to determine the optimum values for the two remaining controller settings. It is shown that the introduction of integral control allows an increase in the proportional control factor for the same degree of damping of the first normal mode of oscillation.Experimental results are given showing how an electronic simulator for process control has been used to confirm the results of the analyses and to assist in determining the optimum settings for integral control.For completeness, a brief description of an automatic-process-control system with definitions of the terms used is included in the paper.