Abstract: Printed wiring board testing apparatus comprising means for optically scanning a board to be tested in orthogonal X-Y axes, with each Y-direction scanning a narrow strip w in the X direction. Means for processing the signals provides signals representative of the characteristics of the board along the strip and for storing the signals. Control means causes the scanning means repeatedly to scan the board in Y direction and to step it a predetermined amount in X direction at each scan end until a required board area board has been scanned and the processing means has processed groups of signals. The processing means identifies common areas of each conductive track, so finally the storage means contains information representative of the co-ordinates of a plurality of datum areas and of the datum areas which are interconnected.