Abstract: A monitoring system for monitoring the axial position of a rotating shaft includes a phonic wheel which is mounted coaxially to the shaft for rotation therewith. The phonic wheel includes a circumferential row of teeth. The system further includes a sensor configured to detect the passage of the row of teeth by generating an alternating measurement signal. The teeth are configured around the row such that the teeth contribute a component to the alternating measurement signal. The teeth are tapered in an axial direction of the wheel and the sensor is positioned relative to the phonic wheel such that, in use, axial displacement of the shaft causes the signal generated by the sensor to vary either or both of the amplitude and the pulse width of the component, whereby the axial position of the shaft can be monitored.