Abstract: A monitoring system for monitoring the axial position of a rotating shaft is provided. The system includes first and second, axially adjacent phonic wheels which are mounted coaxially to the shaft for rotation therewith. The first and second phonic wheels respectively have first and second circumferential rows of detectable features. The system further includes a sensor configured to detect the passage of the first row of detectable features by generating a first alternating measurement signal component, and to detect the passage of the second row of detectable features by generating a second alternating measurement signal component.