Torque, speed, and power are the deŸning parameters of rotating machinery. œe ability to accurately measure these quantities is essential for determining a machine’s e¬ciency and to establish operating regimes that are safe and conducive to long and reliable service. On-line measurements of these quantities enable real-time control, help to ensure consistency in product quality, and can provide early indications of impending problems. Torque and power measurements are used in testing new designs of engines, motors, pumps, and other rotating machines and in the development of new machine components. Torque measurements are used routinely for controlling and verifying the tightness of threaded fasteners. Starting from basic concepts, this chapter describes the various methods and devices currently employed for the measurement of torque and power; the measurement of speed, or more precisely, angular velocity, is discussed elsewhere in this handbook .