Shaft cracks are critical faults in rotating machinery that can lead to catastrophic failure if undetected. Vibration analysis provides a powerful tool for early crack detection during machine operation. Characteristic vibration responses include changes in 1X and 2X amplitude and phase, reduction in critical speed, and internal loop orbit shape [1–3]. In some cases, strong 3X components are also observed as discussed in [4–6]. This article reviews the vibration mechanisms as