On-line crack detection and crack depth measurements have been carried out using a novel sensor. The sensor is based on an advanced alternating current potential difference (ACPD) technique. This sensor makes use of a new technology called current focusing (CF) which allows the current distribution on the surface to be selectively confined to a precise region (e.g., in a high stress gradient domain). The use of AC currents in conjunction with the current focusing technique results in having a three — dimensional control of the current density distribution in a solid. As a result, the technique can be used on large structures as well as on standard specimens. The associated advanced electronics, having variable frequency capabilities (500 Hz to 240 kHz) and variable currents up to 5 Amperes has been designed and will be briefly described. To assess the performance of the system and ACPD/CF technologies, a fatigue testing program was carried out on single edge notch specimens made of Al 7075 alloys. Naturally initiated fatigue cracks in the range of 200 μm to 6mm have been detected. Potential difference versus crack depth calibrations curves were obtained for various combinations of currents and frequencies with a reliability better than 99%. Furthermore, off-line measurements have been carried out to assess the applicability of the technique for NDI applications.