A number of investigators have developed acoustic methods for measurement and determination of crack length. These methods have generally fallen into one of three categories, determination of crack area, time of flight to determine crack length, or recognition of the crack tip by changes in the signal response. These methods have been successful in location of the crack tip to within ± 0.5 mm. In all cases, however, it was necessary for the crack length or the crack-tip position to be determined with the crack in a fully open state in order to remove the effects of crack closure. Recent work has developed acoustic scanning techniques and subsequent analysis to the point where the same accuracy of ± 0.5 mm is now possible through scanning of the crack in an unloaded condition with closure accounted for. A review of the previous methods will be given together with an explanation of the advances in scanning technique and analysis that have allowed this simplification to occur.