The ACPD method is probably the most versatile of all the commercially available NDE techniques. However as applications of such systems increase so does the awareness of serious limitations in present ACPD knowledge. In particular high local crack and notch tip strains can have a marked effect on ACPD response leading to substantial errors in estimates of crackdepth and growth rates. In the present study an investigation has been undertaken into the influence of elastic/plastic notch tip strain on the response of ACPD crack monitoring systems. Experimental work has been undertaken to produce data on the ACPD response observed in two magnetically contrasting materials (EN1A mild steel, NE8 aluminium alloy) using a series of V and U notched bend specimens. An extensive elastic/plastic finite element analysis was conducted to accurately determine the different notch tip strain fields for both materials. A fundamental study was also undertaken into the influence of strain on the electrical resistivity and relative magnetic permeability, the two material parameters governing the ACPD response. The information obtained from the investigations together with results from the FE analysis has made it possible to understand and quantify the influence of elastic/plastic deformation on ACPD response. An electric field model has been successfully developed to explain and predict the effect of increasing strain on the ACPD response in materials where the skin effect is strong. Results have also shown the inapplicability of the compensation method of crack monitoring when levels of plasticity are appreciable and an alternative method has been proposed.