The wide range of applications for single input single output controllers have encouraged interest in monitoring their performance. Over the past two decades researchers in the area have found many performance enhancement opportunities by applying these techniques. These are most evident in large operational plants with hundreds of controllers being monitored at the same time. Early performance measures were based on minimum variance control as a benchmark for controller performance Many other procedures have since emerged that have improved the level of accuracy in these performance measures. In addition, these improvements made it easier to implement control loop performance monitoring in large industrial settings. Performance indices that address specific control objectives such as steady state operations, set-point change, and disturbance rejection were developed. Other indices focused on addressing specific problems such as detection of controller oscillation and control valve problems. Furthermore, general performance settings for large scale implementation were identified.