Main features of a performance monitoring systems operating on loops of refinery plants are illustrated together with examples of application and achieved results. The system analyses data recorded by the DCS during routine operation and originates verdicts about performance of control loops; also indications of causes of low performance and different strategies to adopt are given (retuning, valve maintenance, upstream actions). The system architecture is firstly illustrated, with characteristics of modules which accomplish different tasks of data acquisition and transfer, system configuration and priority assignment, performance analysis and verdicts emission, database query and operator support. Examples of field validation are then presented, with illustration of loop performance before and after actions suggested by the monitoring system. A synthesis of main techniques adopted in the system is finally presented.