Abstract: The invention provides a new apparatus for the detection, measurement and display of roll and roll bearing eccentricity in a rolling mill, and also an apparatus for correction of such eccentricities. The apparatus comprises individual narrow band-pass filter modules, each dedicated to the detection and development of a signal corresponding to one eccentricity, such as fundamental back-up roll eccentricity, fundamental work roll eccentricity and fundamental bearing eccentricity, or any individual harmonics of the fundamentals that are of sufficient value to merit display and corrective action. The resultant signals from the different modules are summed and applied to a display, or employed to take corrective action. Such dedicated modules can be made relatively cheaply and are stable requiring little maintenance, as compared to the more complex equipment proposed hitherto, requiring shaft encoders and/or high speed computers.