A non-intrusive torsional vibration method for monitoring and tracking small changes in crack growth of shafts is presented in this paper. This method resolves and tracks characteristic changes in the natural torsional vibration frequencies that are associated with shaft crack propagation. While this technique is being applied to reactor coolant pumps (RCPs) it is generally applicable to any type of rotating equipment, including drivelines, and can be applied to detecting and tracking changes in blade natural frequencies in gas or steam turbines. This technique was first developed on a laboratory scale rotor test bed to investigate shaft cracking detection techniques under controlled conditions. The test bed provided a mechanism to evaluate sensing technologies and algorithm development. For accurate knowledge of the crack characteristics (crack depth and front), a shaft was seeded with a crack which was then propagated using a three-point bending process. Following each crack growth step, the specimen was evaluated using ultrasonic inspection techniques for crack characterization. After inspection, the shaft was inserted in the rotor test bed for analysis of shaft torsional vibration features. Following success in detecting and tracking crack growth on the test bed, this process was then take to a much bigger machine for verification. In the summer of 2004, the Applied Research Laboratory, along with other EPRI team members (Southern Co. and Jeumont Industrie), instrumented a 41% scale reactor coolant pump in Jeumont, France. On this platform, the team successfully detected and tracked a seeded cut through the shaft. The torsional vibration measurement method has demonstrated the ability to reliably detect changes in the first natural shaft frequency in the range of 0.1 to 0.2 Hz. This technique shows the potential to enable online structural health diagnostics and ultimately prevent shaft or even possibly blade failure due to crack growth.