On-line monitoring techniques have been under development over the past two decades for a variety of applications in process and power industries. In the nuclear power industry, on-line monitoring techniques have been considered for instrument calibration verification as well as equipment and plant condition monitoring. This publication reviews the state of the art in on-line equipment monitoring for nuclear power plant (NPP) applications. Its focus is on instrument calibration verification and extension of calibration intervals of sensors, transmitters and channels. This publication also includes reviews of regulatory positions on this subject and provides examples of implementation of on-line monitoring techniques in NPPs. Furthermore, the in-situ methods for verifying the dynamic performance and response time of process instrumentation are reviewed.