Apparatus for detecting defects in a metallic roll mill, the apparatus comprising: a test head movable along the longitudinal length of an underlying surface of the metallic roll mill, the test head including an eddy current coil and a defect indicator; the apparatus further comprising: a receiver configured to move along a scan path between first and second positions along the length of the roll mill and having recording means configured to record an oscillating signal received from the eddy current coil as it is moved over a plurality of identifiable scan regions or areas of the underlying metallic surface; identification means configured to identify, for the or each scan path, a position of the eddy current coil in relation to each of a plurality of adjacent scan areas in-between the first and second positions; output means configured to generate an output to a display a two-dimensional map representing the oscillating signal received in relation to each scan region or area; signal processing means configured to generate a discrete frequency representation of the oscillating signal for a given scan area based on a frequency or number of oscillations received in each scan area; wherein the apparatus further comprises defect identifying means for identifying automatically from a part of the oscillating signal representation, a predetermined characteristic indicative of the presence of a defect on the display in relation to its position on the underlying metallic surface; characterised in that the defect indicator is responsive to user selection of part of the signal representation of a circumferential defect and is configured to indicate a position on the metallic surface of the metallic roll mill from where the selected part of the signal is received. a receiver configured to move along a scan path between first and second positions along the length of the roll mill and having recording means configured to record an oscillating signal received from the eddy current coil as it is moved over a plurality of identifiable scan regions or areas of the underlying metallic surface; identification means configured to identify, for the or each scan path, a position of the eddy current coil in relation to each of a plurality of adjacent scan areas in-between the first and second positions; output means configured to generate an output to a display a two-dimensional map representing the oscillating signal received in relation to each scan region or area; signal processing means configured to generate a discrete frequency representation of the oscillating signal for a given scan area based on a frequency or number of oscillations received in each scan area; wherein the apparatus further comprises defect identifying means for identifying automatically from a part of the oscillating signal representation, a predetermined characteristic indicative of the presence of a defect on the display in relation to its position on the underlying metallic surface; characterised in that the defect indicator is responsive to user selection of part of the signal representation of a circumferential defect and is configured to indicate a position on the metallic surface of the metallic roll mill from where the selected part of the signal is received.